diff --git a/.codespell_ignore b/.codespell_ignore
index 0bd55bee31ec..3815d77f9a07 100644
--- a/.codespell_ignore
+++ b/.codespell_ignore
@@ -4,11 +4,12 @@
 
 atleast
 circularly
+compiletime
 copyable
 crate
 crossreference
 falsy
 inout
+parameteras
 pullrequest
 statics
-compiletime
diff --git a/toolchain/diagnostics/diagnostic_kind.def b/toolchain/diagnostics/diagnostic_kind.def
index ff4bfefa223f..97889bd1fe80 100644
--- a/toolchain/diagnostics/diagnostic_kind.def
+++ b/toolchain/diagnostics/diagnostic_kind.def
@@ -53,7 +53,6 @@ CARBON_DIAGNOSTIC_KIND(BinaryOperatorRequiresWhitespace)
 CARBON_DIAGNOSTIC_KIND(ExpectedCloseParen)
 CARBON_DIAGNOSTIC_KIND(ExpectedCodeBlock)
 CARBON_DIAGNOSTIC_KIND(ExpectedExpression)
-CARBON_DIAGNOSTIC_KIND(ExpectedFunctionParams)
 CARBON_DIAGNOSTIC_KIND(ExpectedIdentifierAfterDot)
 CARBON_DIAGNOSTIC_KIND(ExpectedParameterName)
 CARBON_DIAGNOSTIC_KIND(ExpectedParenAfter)
@@ -83,6 +82,8 @@ CARBON_DIAGNOSTIC_KIND(ExpectedInNotColon)
 CARBON_DIAGNOSTIC_KIND(ExpectedDeclarationName)
 CARBON_DIAGNOSTIC_KIND(ExpectedDeclarationSemiOrDefinition)
 CARBON_DIAGNOSTIC_KIND(MethodImplNotAllowed)
+CARBON_DIAGNOSTIC_KIND(ParametersRequiredByIntroducer)
+CARBON_DIAGNOSTIC_KIND(ParametersRequiredByDeduced)
 
 // ============================================================================
 // Semantics diagnostics
diff --git a/toolchain/parser/parser.cpp b/toolchain/parser/parser.cpp
index 5d9f17af50b4..5e58fe50fb9a 100644
--- a/toolchain/parser/parser.cpp
+++ b/toolchain/parser/parser.cpp
@@ -22,9 +22,6 @@ CARBON_DIAGNOSTIC(
     OperatorRequiresParentheses, Error,
     "Parentheses are required to disambiguate operator precedence.");
 
-CARBON_DIAGNOSTIC(ExpectedParenAfter, Error, "Expected `(` after `{0}`.",
-                  TokenKind);
-
 CARBON_DIAGNOSTIC(ExpectedSemiAfterExpression, Error,
                   "Expected `;` after expression.");
 
@@ -127,6 +124,8 @@ auto Parser::ConsumeAndAddOpenParen(TokenizedBuffer::Token default_token,
   if (auto open_paren = ConsumeIf(TokenKind::OpenParen)) {
     AddLeafNode(start_kind, *open_paren, /*has_error=*/false);
   } else {
+    CARBON_DIAGNOSTIC(ExpectedParenAfter, Error, "Expected `(` after `{0}`.",
+                      TokenKind);
     emitter_->Emit(*position_, ExpectedParenAfter,
                    tokens_->GetKind(default_token));
     AddLeafNode(start_kind, default_token, /*has_error=*/true);
@@ -766,6 +765,53 @@ auto Parser::HandleCodeBlockFinishState() -> void {
   }
 }
 
+auto Parser::HandleDeclarationNameAndParams(bool params_required) -> void {
+  auto state = PopState();
+
+  if (!ConsumeAndAddLeafNodeIf(TokenKind::Identifier,
+                               ParseNodeKind::DeclaredName)) {
+    emitter_->Emit(*position_, ExpectedDeclarationName,
+                   tokens_->GetKind(state.token));
+    ReturnErrorOnState();
+    return;
+  }
+
+  if (PositionIs(TokenKind::OpenSquareBracket)) {
+    PushState(ParserState::DeclarationNameAndParamsAfterDeduced);
+    PushState(ParserState::ParameterListAsDeduced);
+  } else if (PositionIs(TokenKind::OpenParen)) {
+    PushState(ParserState::ParameterListAsRegular);
+  } else if (params_required) {
+    CARBON_DIAGNOSTIC(ParametersRequiredByIntroducer, Error,
+                      "`{0}` requires a `(` for parameters.", TokenKind);
+    emitter_->Emit(*position_, ParametersRequiredByIntroducer,
+                   tokens_->GetKind(state.token));
+    ReturnErrorOnState();
+  }
+}
+
+auto Parser::HandleDeclarationNameAndParamsAsOptionalState() -> void {
+  HandleDeclarationNameAndParams(/*params_required=*/false);
+}
+
+auto Parser::HandleDeclarationNameAndParamsAsRequiredState() -> void {
+  HandleDeclarationNameAndParams(/*params_required=*/true);
+}
+
+auto Parser::HandleDeclarationNameAndParamsAfterDeducedState() -> void {
+  PopAndDiscardState();
+
+  if (PositionIs(TokenKind::OpenParen)) {
+    PushState(ParserState::ParameterListAsRegular);
+  } else {
+    CARBON_DIAGNOSTIC(
+        ParametersRequiredByDeduced, Error,
+        "A `(` for parameters is required after deduced parameters.");
+    emitter_->Emit(*position_, ParametersRequiredByDeduced);
+    ReturnErrorOnState();
+  }
+}
+
 auto Parser::HandleDeclarationScopeLoopState() -> void {
   // This maintains the current state unless we're at the end of the scope.
 
@@ -807,35 +853,6 @@ auto Parser::HandleDeclarationScopeLoopState() -> void {
   }
 }
 
-auto Parser::HandleDeducedParameterState() -> void {
-  PopAndDiscardState();
-
-  PushState(ParserState::DeducedParameterFinish);
-  PushState(ParserState::PatternAsDeducedParameter);
-}
-
-auto Parser::HandleDeducedParameterFinishState() -> void {
-  auto state = PopState();
-
-  if (state.has_error) {
-    ReturnErrorOnState();
-  }
-
-  if (ConsumeListToken(ParseNodeKind::ParameterListComma,
-                       TokenKind::CloseSquareBracket,
-                       state.has_error) == ListTokenKind::Comma) {
-    PushState(ParserState::DeducedParameter);
-  }
-}
-
-auto Parser::HandleDeducedParameterListFinishState() -> void {
-  auto state = PopState();
-
-  AddNode(ParseNodeKind::DeducedParameterList,
-          ConsumeChecked(TokenKind::CloseSquareBracket), state.subtree_start,
-          state.has_error);
-}
-
 auto Parser::HandleDesignator(bool as_struct) -> void {
   auto state = PopState();
 
@@ -1084,87 +1101,13 @@ auto Parser::HandleFunctionIntroducerState() -> void {
 
   AddLeafNode(ParseNodeKind::FunctionIntroducer, Consume());
 
-  if (!ConsumeAndAddLeafNodeIf(TokenKind::Identifier,
-                               ParseNodeKind::DeclaredName)) {
-    emitter_->Emit(*position_, ExpectedDeclarationName, TokenKind::Fn);
-    // TODO: We could change the lexer to allow us to synthesize certain
-    // kinds of tokens and try to "recover" here, but unclear that this is
-    // really useful.
-    HandleDeclarationError(state, ParseNodeKind::FunctionDeclaration,
-                           /*skip_past_likely_end=*/true);
-    return;
-  }
-
-  // Proceed to the same state regardless of whether there's a deduced
-  // parameter list.
-  state.state = ParserState::FunctionAfterDeducedParameterList;
+  state.state = ParserState::FunctionAfterParameters;
   PushState(state);
-
-  if (!PositionIs(TokenKind::OpenSquareBracket)) {
-    return;
-  }
-
-  // Parse the deduced parameter list as its own subtree.
-  PushState(ParserState::DeducedParameterListFinish);
-  AddLeafNode(ParseNodeKind::DeducedParameterListStart, Consume());
-
-  if (!PositionIs(TokenKind::CloseSquareBracket)) {
-    PushState(ParserState::DeducedParameter);
-  }
-}
-
-auto Parser::HandleFunctionAfterDeducedParameterListState() -> void {
-  auto state = PopState();
-
-  if (!PositionIs(TokenKind::OpenParen)) {
-    CARBON_DIAGNOSTIC(ExpectedFunctionParams, Error,
-                      "Expected `(` after function name.");
-    emitter_->Emit(*position_, ExpectedFunctionParams);
-    HandleDeclarationError(state, ParseNodeKind::FunctionDeclaration,
-                           /*skip_past_likely_end=*/true);
-    return;
-  }
-
-  // Parse the parameter list as its own subtree; once that pops, resume
-  // function parsing.
-  state.state = ParserState::FunctionAfterParameterList;
+  state.state = ParserState::DeclarationNameAndParamsAsRequired;
   PushState(state);
-  PushState(ParserState::FunctionParameterListFinish);
-  AddLeafNode(ParseNodeKind::ParameterListStart, Consume());
-
-  if (!PositionIs(TokenKind::CloseParen)) {
-    PushState(ParserState::FunctionParameter);
-  }
 }
 
-auto Parser::HandleFunctionParameterState() -> void {
-  PopAndDiscardState();
-
-  PushState(ParserState::FunctionParameterFinish);
-  PushState(ParserState::PatternAsFunctionParameter);
-}
-
-auto Parser::HandleFunctionParameterFinishState() -> void {
-  auto state = PopState();
-
-  if (state.has_error) {
-    ReturnErrorOnState();
-  }
-
-  if (ConsumeListToken(ParseNodeKind::ParameterListComma, TokenKind::CloseParen,
-                       state.has_error) == ListTokenKind::Comma) {
-    PushState(ParserState::FunctionParameter);
-  }
-}
-
-auto Parser::HandleFunctionParameterListFinishState() -> void {
-  auto state = PopState();
-
-  AddNode(ParseNodeKind::ParameterList, ConsumeChecked(TokenKind::CloseParen),
-          state.subtree_start, state.has_error);
-}
-
-auto Parser::HandleFunctionAfterParameterListState() -> void {
+auto Parser::HandleFunctionAfterParametersState() -> void {
   auto state = PopState();
 
   // Regardless of whether there's a return type, we'll finish the signature.
@@ -1219,8 +1162,10 @@ auto Parser::HandleFunctionSignatureFinishState() -> void {
       break;
     }
     default: {
-      emitter_->Emit(*position_, ExpectedDeclarationSemiOrDefinition,
-                     TokenKind::Fn);
+      if (!state.has_error) {
+        emitter_->Emit(*position_, ExpectedDeclarationSemiOrDefinition,
+                       TokenKind::Fn);
+      }
       // Only need to skip if we've not already found a new line.
       bool skip_past_likely_end =
           tokens_->GetLine(*position_) == tokens_->GetLine(state.token);
@@ -1316,6 +1261,93 @@ auto Parser::HandlePackageState() -> void {
           /*has_error=*/false);
 }
 
+auto Parser::HandleParameter(ParserState pattern_state,
+                             ParserState finish_state) -> void {
+  PopAndDiscardState();
+
+  PushState(finish_state);
+  PushState(pattern_state);
+}
+
+auto Parser::HandleParameterAsDeducedState() -> void {
+  HandleParameter(ParserState::PatternAsDeducedParameter,
+                  ParserState::ParameterFinishAsDeduced);
+}
+
+auto Parser::HandleParameterAsRegularState() -> void {
+  HandleParameter(ParserState::PatternAsParameter,
+                  ParserState::ParameterFinishAsRegular);
+}
+
+auto Parser::HandleParameterFinish(TokenKind close_token,
+                                   ParserState param_state) -> void {
+  auto state = PopState();
+
+  if (state.has_error) {
+    ReturnErrorOnState();
+  }
+
+  if (ConsumeListToken(ParseNodeKind::ParameterListComma, close_token,
+                       state.has_error) == ListTokenKind::Comma) {
+    PushState(param_state);
+  }
+}
+
+auto Parser::HandleParameterFinishAsDeducedState() -> void {
+  HandleParameterFinish(TokenKind::CloseSquareBracket,
+                        ParserState::ParameterAsDeduced);
+}
+
+auto Parser::HandleParameterFinishAsRegularState() -> void {
+  HandleParameterFinish(TokenKind::CloseParen, ParserState::ParameterAsRegular);
+}
+
+auto Parser::HandleParameterList(ParseNodeKind parse_node_kind,
+                                 TokenKind open_token_kind,
+                                 TokenKind close_token_kind,
+                                 ParserState param_state,
+                                 ParserState finish_state) -> void {
+  PopAndDiscardState();
+
+  PushState(finish_state);
+  AddLeafNode(parse_node_kind, ConsumeChecked(open_token_kind));
+
+  if (!PositionIs(close_token_kind)) {
+    PushState(param_state);
+  }
+}
+
+auto Parser::HandleParameterListAsDeducedState() -> void {
+  HandleParameterList(
+      ParseNodeKind::DeducedParameterListStart, TokenKind::OpenSquareBracket,
+      TokenKind::CloseSquareBracket, ParserState::ParameterAsDeduced,
+      ParserState::ParameterListFinishAsDeduced);
+}
+
+auto Parser::HandleParameterListAsRegularState() -> void {
+  HandleParameterList(ParseNodeKind::ParameterListStart, TokenKind::OpenParen,
+                      TokenKind::CloseParen, ParserState::ParameterAsRegular,
+                      ParserState::ParameterListFinishAsRegular);
+}
+
+auto Parser::HandleParameterListFinish(ParseNodeKind parse_node_kind,
+                                       TokenKind token_kind) -> void {
+  auto state = PopState();
+
+  AddNode(parse_node_kind, ConsumeChecked(token_kind), state.subtree_start,
+          state.has_error);
+}
+
+auto Parser::HandleParameterListFinishAsDeducedState() -> void {
+  HandleParameterListFinish(ParseNodeKind::DeducedParameterList,
+                            TokenKind::CloseSquareBracket);
+}
+
+auto Parser::HandleParameterListFinishAsRegularState() -> void {
+  HandleParameterListFinish(ParseNodeKind::ParameterList,
+                            TokenKind::CloseParen);
+}
+
 auto Parser::HandleParenCondition(ParseNodeKind start_kind,
                                   ParserState finish_state) -> void {
   auto state = PopState();
@@ -1489,7 +1521,7 @@ auto Parser::HandlePatternAsDeducedParameterState() -> void {
   HandlePattern(PatternKind::DeducedParameter);
 }
 
-auto Parser::HandlePatternAsFunctionParameterState() -> void {
+auto Parser::HandlePatternAsParameterState() -> void {
   HandlePattern(PatternKind::Parameter);
 }
 
@@ -1733,17 +1765,39 @@ auto Parser::HandleStatementWhileBlockFinishState() -> void {
 }
 
 auto Parser::HandleTypeIntroducer(ParseNodeKind introducer_kind,
-                                  ParseNodeKind declaration_kind,
-                                  ParseNodeKind definition_start_kind,
-                                  ParserState definition_finish_state) -> void {
+                                  ParserState after_params_state) -> void {
   auto state = PopState();
 
   AddLeafNode(introducer_kind, Consume());
 
-  if (!ConsumeAndAddLeafNodeIf(TokenKind::Identifier,
-                               ParseNodeKind::DeclaredName)) {
-    emitter_->Emit(*position_, ExpectedDeclarationName,
-                   tokens_->GetKind(state.token));
+  state.state = after_params_state;
+  PushState(state);
+  state.state = ParserState::DeclarationNameAndParamsAsOptional;
+  PushState(state);
+}
+
+auto Parser::HandleTypeIntroducerAsClassState() -> void {
+  HandleTypeIntroducer(ParseNodeKind::ClassIntroducer,
+                       ParserState::TypeAfterParamsAsClass);
+}
+
+auto Parser::HandleTypeIntroducerAsInterfaceState() -> void {
+  HandleTypeIntroducer(ParseNodeKind::InterfaceIntroducer,
+                       ParserState::TypeAfterParamsAsInterface);
+}
+
+auto Parser::HandleTypeIntroducerAsNamedConstraintState() -> void {
+  HandleTypeIntroducer(ParseNodeKind::NamedConstraintIntroducer,
+                       ParserState::TypeAfterParamsAsNamedConstraint);
+}
+
+auto Parser::HandleTypeAfterParams(ParseNodeKind declaration_kind,
+                                   ParseNodeKind definition_start_kind,
+                                   ParserState definition_finish_state)
+    -> void {
+  auto state = PopState();
+
+  if (state.has_error) {
     HandleDeclarationError(state, declaration_kind,
                            /*skip_past_likely_end=*/true);
     return;
@@ -1769,25 +1823,22 @@ auto Parser::HandleTypeIntroducer(ParseNodeKind introducer_kind,
           state.has_error);
 }
 
-auto Parser::HandleTypeIntroducerAsClassState() -> void {
-  HandleTypeIntroducer(ParseNodeKind::ClassIntroducer,
-                       ParseNodeKind::ClassDeclaration,
-                       ParseNodeKind::ClassDefinitionStart,
-                       ParserState::TypeDefinitionFinishAsClass);
+auto Parser::HandleTypeAfterParamsAsClassState() -> void {
+  HandleTypeAfterParams(ParseNodeKind::ClassDeclaration,
+                        ParseNodeKind::ClassDefinitionStart,
+                        ParserState::TypeDefinitionFinishAsClass);
 }
 
-auto Parser::HandleTypeIntroducerAsInterfaceState() -> void {
-  HandleTypeIntroducer(ParseNodeKind::InterfaceIntroducer,
-                       ParseNodeKind::InterfaceDeclaration,
-                       ParseNodeKind::InterfaceDefinitionStart,
-                       ParserState::TypeDefinitionFinishAsInterface);
+auto Parser::HandleTypeAfterParamsAsInterfaceState() -> void {
+  HandleTypeAfterParams(ParseNodeKind::InterfaceDeclaration,
+                        ParseNodeKind::InterfaceDefinitionStart,
+                        ParserState::TypeDefinitionFinishAsInterface);
 }
 
-auto Parser::HandleTypeIntroducerAsNamedConstraintState() -> void {
-  HandleTypeIntroducer(ParseNodeKind::NamedConstraintIntroducer,
-                       ParseNodeKind::NamedConstraintDeclaration,
-                       ParseNodeKind::NamedConstraintDefinitionStart,
-                       ParserState::TypeDefinitionFinishAsNamedConstraint);
+auto Parser::HandleTypeAfterParamsAsNamedConstraintState() -> void {
+  HandleTypeAfterParams(ParseNodeKind::NamedConstraintDeclaration,
+                        ParseNodeKind::NamedConstraintDefinitionStart,
+                        ParserState::TypeDefinitionFinishAsNamedConstraint);
 }
 
 auto Parser::HandleTypeDefinitionFinish(ParseNodeKind definition_kind) -> void {
diff --git a/toolchain/parser/parser.h b/toolchain/parser/parser.h
index ef8b064fd6b0..ab15f17d39f0 100644
--- a/toolchain/parser/parser.h
+++ b/toolchain/parser/parser.h
@@ -305,9 +305,30 @@ class Parser {
   // Handles BraceExpressionFinishAs(Type|Value|Unknown).
   auto HandleBraceExpressionFinish(BraceExpressionKind kind) -> void;
 
+  // Handles DeclarationNameAndParamsAs(Optional|Required).
+  auto HandleDeclarationNameAndParams(bool params_required) -> void;
+
   // Handles DesignatorAs.
   auto HandleDesignator(bool as_struct) -> void;
 
+  // Handles ParameterAs(Deduced|Regular).
+  auto HandleParameter(ParserState pattern_state, ParserState finish_state)
+      -> void;
+
+  // Handles ParameterFinishAs(Deduced|Regular).
+  auto HandleParameterFinish(TokenKind close_token, ParserState param_state)
+      -> void;
+
+  // Handles ParameterListAs(Deduced|Regular).
+  auto HandleParameterList(ParseNodeKind parse_node_kind,
+                           TokenKind open_token_kind,
+                           TokenKind close_token_kind, ParserState param_state,
+                           ParserState finish_state) -> void;
+
+  // Handles ParameterListFinishAs(Deduced|Regular).
+  auto HandleParameterListFinish(ParseNodeKind parse_node_kind,
+                                 TokenKind token_kind) -> void;
+
   // Handles ParenConditionAs(If|While)
   auto HandleParenCondition(ParseNodeKind start_kind, ParserState finish_state)
       -> void;
@@ -321,11 +342,15 @@ class Parser {
   // Handles the `;` after a keyword statement.
   auto HandleStatementKeywordFinish(ParseNodeKind node_kind) -> void;
 
-  // Handles processing of a type's `<introducer> <name> {`.
+  // Handles processing of a type's introducer.
   auto HandleTypeIntroducer(ParseNodeKind introducer_kind,
-                            ParseNodeKind declaration_kind,
-                            ParseNodeKind definition_start_kind,
-                            ParserState definition_finish_state) -> void;
+                            ParserState after_params_state) -> void;
+
+  // Handles processing after params, deciding whether it's a declaration or
+  // definition.
+  auto HandleTypeAfterParams(ParseNodeKind declaration_kind,
+                             ParseNodeKind definition_start_kind,
+                             ParserState definition_finish_state) -> void;
 
   // Handles parsing after the declaration scope of a type.
   auto HandleTypeDefinitionFinish(ParseNodeKind definition_kind) -> void;
diff --git a/toolchain/parser/parser_state.def b/toolchain/parser/parser_state.def
index 81ddd726344d..fa1200c4d2c2 100644
--- a/toolchain/parser/parser_state.def
+++ b/toolchain/parser/parser_state.def
@@ -127,6 +127,26 @@ CARBON_PARSER_STATE(CodeBlock)
 //   (state done)
 CARBON_PARSER_STATE(CodeBlockFinish)
 
+// Handles a general declaration name and parameters, such as `Foo[...](...)`.
+//
+// If `OpenSquareBracket`:
+//   1. ParameterListAsDeduced
+//   2. DeclarationNameAndParamsAfterDeduced
+// If `OpenParen`:
+//   1. ParameterListAsRegular
+// Else:
+//   (state done)
+CARBON_PARSER_STATE_VARIANTS2(DeclarationNameAndParams, Optional, Required)
+
+// Handles regular parameters such as `(...)` for the general declaration case.
+// Only used after deduced parameters.
+//
+// If `OpenParen`:
+//   1. ParameterListAsRegular
+// Else:
+//   (state done)
+CARBON_PARSER_STATE(DeclarationNameAndParamsAfterDeduced)
+
 // Handles processing of a declaration scope. Things like fn, class, interface,
 // and so on.
 //
@@ -153,28 +173,6 @@ CARBON_PARSER_STATE(CodeBlockFinish)
 //   1. DeclarationScopeLoop
 CARBON_PARSER_STATE(DeclarationScopeLoop)
 
-// Starts deduced parameter processing.
-//
-// Always:
-//   1. PatternAsDeducedParameter
-//   2. DeducedParameterFinish
-CARBON_PARSER_STATE(DeducedParameter)
-
-// Finishes deduced parameter processing, including `,`. If there are more
-// parameters, enqueues another parameter processing state.
-//
-// If `Comma` without `CloseSquareBracket`:
-//   1. DeducedParameter
-// Else:
-//   (state done)
-CARBON_PARSER_STATE(DeducedParameterFinish)
-
-// Handles processing of a deduced parameter list `]`.
-//
-// Always:
-//   (state done)
-CARBON_PARSER_STATE(DeducedParameterListFinish)
-
 // Handles a designator expression, such as `.z` in `x.(y.z)`.
 //
 // Always:
@@ -251,55 +249,15 @@ CARBON_PARSER_STATE(ExpressionLoopForPrefix)
 //   (state done)
 CARBON_PARSER_STATE(ExpressionStatementFinish)
 
-// Handles processing of a function's `fn <name>(`, and enqueues parameter list
-// handling.
+// Handles a function's introducer.
 //
 // If invalid:
 //   (state done)
-// If deduced parameters:
-//   1. DeducedParameter
-//   2. DeducedParameterListFinish
-//   3. FunctionAfterDeducedParameterList
 // Else:
-//   1. FunctionAfterDeducedParameterList
+//   1. DeclarationNameAndParamsAsRequired
+//   2. FunctionAfterParameters
 CARBON_PARSER_STATE(FunctionIntroducer)
 
-// Handles processing of a functions' syntax after the deduced parameter lists's
-// `]`. This applies only to interfaces and classes.
-//
-// If invalid:
-//   (state done)
-// If parenthesized parameters:
-//   1. FunctionParameter
-//   2. FunctionParameterListFinish
-//   3. FunctionAfterParameterList
-// Else:
-//   1. FunctionParameterListFinish
-//   2. FunctionAfterParameterList
-CARBON_PARSER_STATE(FunctionAfterDeducedParameterList)
-
-// Starts function parameter processing.
-//
-// Always:
-//   1. PatternAsFunctionParameter
-//   2. FunctionParameterFinish
-CARBON_PARSER_STATE(FunctionParameter)
-
-// Finishes function parameter processing, including `,`. If there are more
-// parameters, enqueues another parameter processing state.
-//
-// If `Comma` without `CloseParen`:
-//   1. FunctionParameter
-// Else:
-//   (state done)
-CARBON_PARSER_STATE(FunctionParameterFinish)
-
-// Handles processing of a function's parameter list `)`.
-//
-// Always:
-//   (state done)
-CARBON_PARSER_STATE(FunctionParameterListFinish)
-
 // Handles processing of a function's syntax after `)`, primarily the
 // possibility a `->` return type is there. Always enqueues signature finish
 // handling.
@@ -310,7 +268,7 @@ CARBON_PARSER_STATE(FunctionParameterListFinish)
 //   3. FunctionSignatureFinish
 // Else:
 //   1. FunctionSignatureFinish
-CARBON_PARSER_STATE(FunctionAfterParameterList)
+CARBON_PARSER_STATE(FunctionAfterParameters)
 
 // Finishes a function return type.
 //
@@ -342,6 +300,37 @@ CARBON_PARSER_STATE(FunctionDefinitionFinish)
 //   (state done)
 CARBON_PARSER_STATE(Package)
 
+// Starts deduced parameter processing.
+//
+// Always:
+//   1. PatternAs(DeducedParameter|Parameter)
+//   2. ParameterFinishAs(Deduced|Regular)
+CARBON_PARSER_STATE_VARIANTS2(Parameter, Deduced, Regular)
+
+// Finishes deduced parameter processing, including `,`. If there are more
+// parameters, enqueues another parameter processing state.
+//
+// If `Comma` without the list close token:
+//   1. ParameterAs(Deduced|Regular)
+// Else:
+//   (state done)
+CARBON_PARSER_STATE_VARIANTS2(ParameterFinish, Deduced, Regular)
+
+// Handles processing of a parameter list `[` or `(`.
+//
+// If the list close token:
+//   1. ParameterListFinishAs(Deduced|Regular)
+// Else:
+//   1. ParameterAs(Deduced|Regular)
+//   2. ParameterListFinishAs(Deduced|Regular)
+CARBON_PARSER_STATE_VARIANTS2(ParameterList, Deduced, Regular)
+
+// Handles processing of a parameter list `]` or `)`.
+//
+// Always:
+//   (state done)
+CARBON_PARSER_STATE_VARIANTS2(ParameterListFinish, Deduced, Regular)
+
 // Handles the processing of a `(condition)` up through the expression.
 //
 // Always:
@@ -394,8 +383,7 @@ CARBON_PARSER_STATE_VARIANTS2(ParenExpressionFinish, Normal, Tuple)
 //   2. PatternFinish
 // Else:
 //   1. PatternFinish
-CARBON_PARSER_STATE_VARIANTS3(Pattern, DeducedParameter, FunctionParameter,
-                              Variable)
+CARBON_PARSER_STATE_VARIANTS3(Pattern, DeducedParameter, Parameter, Variable)
 
 // Handles `addr` in a pattern.
 //
@@ -563,11 +551,17 @@ CARBON_PARSER_STATE(StatementWhileBlockFinish)
 //
 // Always:
 //   (state done)
-CARBON_PARSER_STATE(TypeDefinitionFinishAsClass)
-CARBON_PARSER_STATE(TypeDefinitionFinishAsInterface)
-CARBON_PARSER_STATE(TypeDefinitionFinishAsNamedConstraint)
+CARBON_PARSER_STATE_VARIANTS3(TypeDefinitionFinish, Class, Interface,
+                              NamedConstraint)
 
-// Handles processing of a type's `<introducer> <name> [;{]`.
+// Handles processing of a type's introducer.
+//
+// Always:
+//   1. DeclarationNameAndParamsAsOptional
+//   2. TypeAfterParamsAs(Class|Interface|NamedConstraint)
+CARBON_PARSER_STATE_VARIANTS3(TypeIntroducer, Class, Interface, NamedConstraint)
+
+// Handles processing of a type after its optional parameters.
 //
 // If `Semi`:
 //   (state done)
@@ -576,9 +570,8 @@ CARBON_PARSER_STATE(TypeDefinitionFinishAsNamedConstraint)
 //   2. TypeDefinitionFinishAs(Class|Interface|NamedConstraint)
 // Else:
 //   (state done)
-CARBON_PARSER_STATE(TypeIntroducerAsClass)
-CARBON_PARSER_STATE(TypeIntroducerAsInterface)
-CARBON_PARSER_STATE(TypeIntroducerAsNamedConstraint)
+CARBON_PARSER_STATE_VARIANTS3(TypeAfterParams, Class, Interface,
+                              NamedConstraint)
 
 // Handles the start of a `var`.
 //
diff --git a/toolchain/parser/testdata/function/declaration/deduced_one.carbon b/toolchain/parser/testdata/function/declaration/deduced_empty.carbon
similarity index 76%
rename from toolchain/parser/testdata/function/declaration/deduced_one.carbon
rename to toolchain/parser/testdata/function/declaration/deduced_empty.carbon
index b7534ee18659..91b161775a6d 100644
--- a/toolchain/parser/testdata/function/declaration/deduced_one.carbon
+++ b/toolchain/parser/testdata/function/declaration/deduced_empty.carbon
@@ -8,14 +8,11 @@
 // CHECK:STDOUT:   {kind: 'FunctionIntroducer', text: 'fn'},
 // CHECK:STDOUT:   {kind: 'DeclaredName', text: 'foo'},
 // CHECK:STDOUT:     {kind: 'DeducedParameterListStart', text: '['},
-// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'a'},
-// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
-// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
-// CHECK:STDOUT:   {kind: 'DeducedParameterList', text: ']', subtree_size: 5},
+// CHECK:STDOUT:   {kind: 'DeducedParameterList', text: ']', subtree_size: 2},
 // CHECK:STDOUT:     {kind: 'ParameterListStart', text: '('},
 // CHECK:STDOUT:   {kind: 'ParameterList', text: ')', subtree_size: 2},
-// CHECK:STDOUT: {kind: 'FunctionDeclaration', text: ';', subtree_size: 10},
+// CHECK:STDOUT: {kind: 'FunctionDeclaration', text: ';', subtree_size: 7},
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT: ]
 
-fn foo[a: i32]();
+fn foo[]();
diff --git a/toolchain/parser/testdata/function/declaration/deduced_one_suffix_comma.carbon b/toolchain/parser/testdata/function/declaration/deduced_one_suffix_comma.carbon
deleted file mode 100644
index 01e130ca7929..000000000000
--- a/toolchain/parser/testdata/function/declaration/deduced_one_suffix_comma.carbon
+++ /dev/null
@@ -1,22 +0,0 @@
-// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
-// Exceptions. See /LICENSE for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-// AUTOUPDATE
-// RUN: %{carbon-run-parser}
-// CHECK:STDOUT: [
-// CHECK:STDOUT:   {kind: 'FunctionIntroducer', text: 'fn'},
-// CHECK:STDOUT:   {kind: 'DeclaredName', text: 'foo'},
-// CHECK:STDOUT:     {kind: 'DeducedParameterListStart', text: '['},
-// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'a'},
-// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
-// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
-// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
-// CHECK:STDOUT:   {kind: 'DeducedParameterList', text: ']', subtree_size: 6},
-// CHECK:STDOUT:     {kind: 'ParameterListStart', text: '('},
-// CHECK:STDOUT:   {kind: 'ParameterList', text: ')', subtree_size: 2},
-// CHECK:STDOUT: {kind: 'FunctionDeclaration', text: ';', subtree_size: 11},
-// CHECK:STDOUT: {kind: 'FileEnd', text: ''},
-// CHECK:STDOUT: ]
-
-fn foo[a: i32,]();
diff --git a/toolchain/parser/testdata/function/declaration/deduced_two.carbon b/toolchain/parser/testdata/function/declaration/deduced_params.carbon
similarity index 100%
rename from toolchain/parser/testdata/function/declaration/deduced_two.carbon
rename to toolchain/parser/testdata/function/declaration/deduced_params.carbon
diff --git a/toolchain/parser/testdata/function/declaration/deduced_six.carbon b/toolchain/parser/testdata/function/declaration/deduced_six.carbon
deleted file mode 100644
index 306c5271ca2c..000000000000
--- a/toolchain/parser/testdata/function/declaration/deduced_six.carbon
+++ /dev/null
@@ -1,41 +0,0 @@
-// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
-// Exceptions. See /LICENSE for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-// AUTOUPDATE
-// RUN: %{carbon-run-parser}
-// CHECK:STDOUT: [
-// CHECK:STDOUT:   {kind: 'FunctionIntroducer', text: 'fn'},
-// CHECK:STDOUT:   {kind: 'DeclaredName', text: 'foo'},
-// CHECK:STDOUT:     {kind: 'DeducedParameterListStart', text: '['},
-// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'a'},
-// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
-// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
-// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
-// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'b'},
-// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
-// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
-// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
-// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'c'},
-// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
-// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
-// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
-// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'd'},
-// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
-// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
-// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
-// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'e'},
-// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
-// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
-// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
-// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'f'},
-// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
-// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
-// CHECK:STDOUT:   {kind: 'DeducedParameterList', text: ']', subtree_size: 25},
-// CHECK:STDOUT:     {kind: 'ParameterListStart', text: '('},
-// CHECK:STDOUT:   {kind: 'ParameterList', text: ')', subtree_size: 2},
-// CHECK:STDOUT: {kind: 'FunctionDeclaration', text: ';', subtree_size: 30},
-// CHECK:STDOUT: {kind: 'FileEnd', text: ''},
-// CHECK:STDOUT: ]
-
-fn foo[a: i32, b: i32, c: i32, d: i32, e: i32, f: i32]();
diff --git a/toolchain/parser/testdata/function/declaration/deduced_two_suffix_comma.carbon b/toolchain/parser/testdata/function/declaration/deduced_two_suffix_comma.carbon
deleted file mode 100644
index 30bebd69cb8a..000000000000
--- a/toolchain/parser/testdata/function/declaration/deduced_two_suffix_comma.carbon
+++ /dev/null
@@ -1,26 +0,0 @@
-// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
-// Exceptions. See /LICENSE for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-// AUTOUPDATE
-// RUN: %{carbon-run-parser}
-// CHECK:STDOUT: [
-// CHECK:STDOUT:   {kind: 'FunctionIntroducer', text: 'fn'},
-// CHECK:STDOUT:   {kind: 'DeclaredName', text: 'foo'},
-// CHECK:STDOUT:     {kind: 'DeducedParameterListStart', text: '['},
-// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'a'},
-// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
-// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
-// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
-// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'b'},
-// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
-// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
-// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
-// CHECK:STDOUT:   {kind: 'DeducedParameterList', text: ']', subtree_size: 10},
-// CHECK:STDOUT:     {kind: 'ParameterListStart', text: '('},
-// CHECK:STDOUT:   {kind: 'ParameterList', text: ')', subtree_size: 2},
-// CHECK:STDOUT: {kind: 'FunctionDeclaration', text: ';', subtree_size: 15},
-// CHECK:STDOUT: {kind: 'FileEnd', text: ''},
-// CHECK:STDOUT: ]
-
-fn foo[a: i32, b: i32,]();
diff --git a/toolchain/parser/testdata/function/declaration/fail_identifier_instead_of_sig.carbon b/toolchain/parser/testdata/function/declaration/fail_identifier_instead_of_sig.carbon
index c54e50d4b4ac..9d8a737e384f 100644
--- a/toolchain/parser/testdata/function/declaration/fail_identifier_instead_of_sig.carbon
+++ b/toolchain/parser/testdata/function/declaration/fail_identifier_instead_of_sig.carbon
@@ -11,5 +11,5 @@
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT: ]
 
-// CHECK:STDERR: {{.*}}/toolchain/parser/testdata/function/declaration/fail_identifier_instead_of_sig.carbon:[[@LINE+1]]:8: Expected `(` after function name.
+// CHECK:STDERR: {{.*}}/toolchain/parser/testdata/function/declaration/fail_identifier_instead_of_sig.carbon:[[@LINE+1]]:8: `fn` requires a `(` for parameters.
 fn foo bar;
diff --git a/toolchain/parser/testdata/function/declaration/fail_missing_deduced_close.carbon b/toolchain/parser/testdata/function/declaration/fail_missing_deduced_close.carbon
index 0f79c7e73646..5eaf8e1c93e7 100644
--- a/toolchain/parser/testdata/function/declaration/fail_missing_deduced_close.carbon
+++ b/toolchain/parser/testdata/function/declaration/fail_missing_deduced_close.carbon
@@ -17,4 +17,4 @@
 // CHECK:STDERR: {{.*}}/toolchain/parser/testdata/function/declaration/fail_missing_deduced_close.carbon:[[@LINE+2]]:7: Closing symbol does not match most recent opening symbol.
 // CHECK:STDERR: {{.*}}/toolchain/parser/testdata/function/declaration/fail_missing_deduced_close.carbon:[[@LINE+1]]:8: Expected parameter declaration.
 fn Div[();
-// CHECK:STDERR: {{.*}}/toolchain/parser/testdata/function/declaration/fail_missing_deduced_close.carbon:[[@LINE+0]]:156: Expected `(` after function name.
+// CHECK:STDERR: {{.*}}/toolchain/parser/testdata/function/declaration/fail_missing_deduced_close.carbon:[[@LINE+0]]:181: A `(` for parameters is required after deduced parameters.
diff --git a/toolchain/parser/testdata/function/declaration/fail_no_sig_or_semi.carbon b/toolchain/parser/testdata/function/declaration/fail_no_sig_or_semi.carbon
index eba6e4312bb7..42ef84ce45a6 100644
--- a/toolchain/parser/testdata/function/declaration/fail_no_sig_or_semi.carbon
+++ b/toolchain/parser/testdata/function/declaration/fail_no_sig_or_semi.carbon
@@ -11,5 +11,5 @@
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT: ]
 
-// CHECK:STDERR: {{.*}}/toolchain/parser/testdata/function/declaration/fail_no_sig_or_semi.carbon:[[@LINE+1]]:7: Expected `(` after function name.
+// CHECK:STDERR: {{.*}}/toolchain/parser/testdata/function/declaration/fail_no_sig_or_semi.carbon:[[@LINE+1]]:7: `fn` requires a `(` for parameters.
 fn foo
diff --git a/toolchain/parser/testdata/function/declaration/params_two.carbon b/toolchain/parser/testdata/function/declaration/params.carbon
similarity index 100%
rename from toolchain/parser/testdata/function/declaration/params_two.carbon
rename to toolchain/parser/testdata/function/declaration/params.carbon
diff --git a/toolchain/parser/testdata/function/declaration/params_one.carbon b/toolchain/parser/testdata/function/declaration/params_one.carbon
deleted file mode 100644
index 21dd3b90d47b..000000000000
--- a/toolchain/parser/testdata/function/declaration/params_one.carbon
+++ /dev/null
@@ -1,19 +0,0 @@
-// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
-// Exceptions. See /LICENSE for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-// AUTOUPDATE
-// RUN: %{carbon-run-parser}
-// CHECK:STDOUT: [
-// CHECK:STDOUT:   {kind: 'FunctionIntroducer', text: 'fn'},
-// CHECK:STDOUT:   {kind: 'DeclaredName', text: 'foo'},
-// CHECK:STDOUT:     {kind: 'ParameterListStart', text: '('},
-// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'a'},
-// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
-// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
-// CHECK:STDOUT:   {kind: 'ParameterList', text: ')', subtree_size: 5},
-// CHECK:STDOUT: {kind: 'FunctionDeclaration', text: ';', subtree_size: 8},
-// CHECK:STDOUT: {kind: 'FileEnd', text: ''},
-// CHECK:STDOUT: ]
-
-fn foo(a: i32);
diff --git a/toolchain/parser/testdata/function/declaration/params_one_suffix_comma.carbon b/toolchain/parser/testdata/function/declaration/params_one_suffix_comma.carbon
deleted file mode 100644
index 9f61a2e41323..000000000000
--- a/toolchain/parser/testdata/function/declaration/params_one_suffix_comma.carbon
+++ /dev/null
@@ -1,20 +0,0 @@
-// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
-// Exceptions. See /LICENSE for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-// AUTOUPDATE
-// RUN: %{carbon-run-parser}
-// CHECK:STDOUT: [
-// CHECK:STDOUT:   {kind: 'FunctionIntroducer', text: 'fn'},
-// CHECK:STDOUT:   {kind: 'DeclaredName', text: 'foo'},
-// CHECK:STDOUT:     {kind: 'ParameterListStart', text: '('},
-// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'a'},
-// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
-// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
-// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
-// CHECK:STDOUT:   {kind: 'ParameterList', text: ')', subtree_size: 6},
-// CHECK:STDOUT: {kind: 'FunctionDeclaration', text: ';', subtree_size: 9},
-// CHECK:STDOUT: {kind: 'FileEnd', text: ''},
-// CHECK:STDOUT: ]
-
-fn foo(a: i32,);
diff --git a/toolchain/parser/testdata/function/declaration/params_six.carbon b/toolchain/parser/testdata/function/declaration/params_six.carbon
deleted file mode 100644
index 672bfa408417..000000000000
--- a/toolchain/parser/testdata/function/declaration/params_six.carbon
+++ /dev/null
@@ -1,39 +0,0 @@
-// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
-// Exceptions. See /LICENSE for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-// AUTOUPDATE
-// RUN: %{carbon-run-parser}
-// CHECK:STDOUT: [
-// CHECK:STDOUT:   {kind: 'FunctionIntroducer', text: 'fn'},
-// CHECK:STDOUT:   {kind: 'DeclaredName', text: 'foo'},
-// CHECK:STDOUT:     {kind: 'ParameterListStart', text: '('},
-// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'a'},
-// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
-// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
-// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
-// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'b'},
-// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
-// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
-// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
-// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'c'},
-// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
-// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
-// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
-// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'd'},
-// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
-// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
-// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
-// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'e'},
-// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
-// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
-// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
-// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'f'},
-// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
-// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
-// CHECK:STDOUT:   {kind: 'ParameterList', text: ')', subtree_size: 25},
-// CHECK:STDOUT: {kind: 'FunctionDeclaration', text: ';', subtree_size: 28},
-// CHECK:STDOUT: {kind: 'FileEnd', text: ''},
-// CHECK:STDOUT: ]
-
-fn foo(a: i32, b: i32, c: i32, d: i32, e: i32, f: i32);
diff --git a/toolchain/parser/testdata/function/declaration/params_two_suffix_comma.carbon b/toolchain/parser/testdata/function/declaration/params_two_suffix_comma.carbon
deleted file mode 100644
index 900f90cc4857..000000000000
--- a/toolchain/parser/testdata/function/declaration/params_two_suffix_comma.carbon
+++ /dev/null
@@ -1,24 +0,0 @@
-// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
-// Exceptions. See /LICENSE for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-// AUTOUPDATE
-// RUN: %{carbon-run-parser}
-// CHECK:STDOUT: [
-// CHECK:STDOUT:   {kind: 'FunctionIntroducer', text: 'fn'},
-// CHECK:STDOUT:   {kind: 'DeclaredName', text: 'foo'},
-// CHECK:STDOUT:     {kind: 'ParameterListStart', text: '('},
-// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'a'},
-// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
-// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
-// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
-// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'b'},
-// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
-// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
-// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
-// CHECK:STDOUT:   {kind: 'ParameterList', text: ')', subtree_size: 10},
-// CHECK:STDOUT: {kind: 'FunctionDeclaration', text: ';', subtree_size: 13},
-// CHECK:STDOUT: {kind: 'FileEnd', text: ''},
-// CHECK:STDOUT: ]
-
-fn foo(a: i32, b: i32,);
diff --git a/toolchain/parser/testdata/generics/deduced_params/empty.carbon b/toolchain/parser/testdata/generics/deduced_params/empty.carbon
new file mode 100644
index 000000000000..177b8827d650
--- /dev/null
+++ b/toolchain/parser/testdata/generics/deduced_params/empty.carbon
@@ -0,0 +1,28 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// RUN: %{carbon-run-parser}
+// CHECK:STDOUT: [
+// CHECK:STDOUT:   {kind: 'ClassIntroducer', text: 'class'},
+// CHECK:STDOUT:   {kind: 'DeclaredName', text: 'Foo'},
+// CHECK:STDOUT:     {kind: 'DeducedParameterListStart', text: '['},
+// CHECK:STDOUT:   {kind: 'DeducedParameterList', text: ']', subtree_size: 2},
+// CHECK:STDOUT:     {kind: 'ParameterListStart', text: '('},
+// CHECK:STDOUT:   {kind: 'ParameterList', text: ')', subtree_size: 2},
+// CHECK:STDOUT: {kind: 'ClassDeclaration', text: ';', subtree_size: 7},
+// CHECK:STDOUT:     {kind: 'InterfaceIntroducer', text: 'interface'},
+// CHECK:STDOUT:     {kind: 'DeclaredName', text: 'Bar'},
+// CHECK:STDOUT:       {kind: 'DeducedParameterListStart', text: '['},
+// CHECK:STDOUT:     {kind: 'DeducedParameterList', text: ']', subtree_size: 2},
+// CHECK:STDOUT:       {kind: 'ParameterListStart', text: '('},
+// CHECK:STDOUT:     {kind: 'ParameterList', text: ')', subtree_size: 2},
+// CHECK:STDOUT:   {kind: 'InterfaceDefinitionStart', text: '{', subtree_size: 7},
+// CHECK:STDOUT: {kind: 'InterfaceDefinition', text: '}', subtree_size: 8},
+// CHECK:STDOUT: {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT: ]
+
+class Foo[]();
+
+interface Bar[]() {}
diff --git a/toolchain/parser/testdata/generics/deduced_params/fail_no_parens.carbon b/toolchain/parser/testdata/generics/deduced_params/fail_no_parens.carbon
new file mode 100644
index 000000000000..7534ec55039a
--- /dev/null
+++ b/toolchain/parser/testdata/generics/deduced_params/fail_no_parens.carbon
@@ -0,0 +1,47 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// RUN: %{not} %{carbon-run-parser}
+// CHECK:STDOUT: [
+// CHECK:STDOUT:   {kind: 'ClassIntroducer', text: 'class'},
+// CHECK:STDOUT:   {kind: 'DeclaredName', text: 'Foo'},
+// CHECK:STDOUT:     {kind: 'DeducedParameterListStart', text: '['},
+// CHECK:STDOUT:   {kind: 'DeducedParameterList', text: ']', subtree_size: 2},
+// CHECK:STDOUT: {kind: 'ClassDeclaration', text: ';', has_error: yes, subtree_size: 5},
+// CHECK:STDOUT:   {kind: 'ClassIntroducer', text: 'class'},
+// CHECK:STDOUT:   {kind: 'DeclaredName', text: 'Foo'},
+// CHECK:STDOUT:     {kind: 'DeducedParameterListStart', text: '['},
+// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'a'},
+// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:   {kind: 'DeducedParameterList', text: ']', subtree_size: 5},
+// CHECK:STDOUT: {kind: 'ClassDeclaration', text: ';', has_error: yes, subtree_size: 8},
+// CHECK:STDOUT:   {kind: 'InterfaceIntroducer', text: 'interface'},
+// CHECK:STDOUT:   {kind: 'DeclaredName', text: 'Bar'},
+// CHECK:STDOUT:     {kind: 'DeducedParameterListStart', text: '['},
+// CHECK:STDOUT:   {kind: 'DeducedParameterList', text: ']', subtree_size: 2},
+// CHECK:STDOUT: {kind: 'InterfaceDeclaration', text: 'interface', has_error: yes, subtree_size: 5},
+// CHECK:STDOUT:   {kind: 'InterfaceIntroducer', text: 'interface'},
+// CHECK:STDOUT:   {kind: 'DeclaredName', text: 'Bar'},
+// CHECK:STDOUT:     {kind: 'DeducedParameterListStart', text: '['},
+// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'a'},
+// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:   {kind: 'DeducedParameterList', text: ']', subtree_size: 5},
+// CHECK:STDOUT: {kind: 'InterfaceDeclaration', text: 'interface', has_error: yes, subtree_size: 8},
+// CHECK:STDOUT: {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT: ]
+
+// CHECK:STDERR: {{.*}}/toolchain/parser/testdata/generics/deduced_params/fail_no_parens.carbon:[[@LINE+1]]:12: A `(` for parameters is required after deduced parameters.
+class Foo[];
+
+// CHECK:STDERR: {{.*}}/toolchain/parser/testdata/generics/deduced_params/fail_no_parens.carbon:[[@LINE+1]]:18: A `(` for parameters is required after deduced parameters.
+class Foo[a: i32];
+
+// CHECK:STDERR: {{.*}}/toolchain/parser/testdata/generics/deduced_params/fail_no_parens.carbon:[[@LINE+1]]:17: A `(` for parameters is required after deduced parameters.
+interface Bar[] {}
+
+// CHECK:STDERR: {{.*}}/toolchain/parser/testdata/generics/deduced_params/fail_no_parens.carbon:[[@LINE+1]]:23: A `(` for parameters is required after deduced parameters.
+interface Bar[a: i32] {}
diff --git a/toolchain/parser/testdata/generics/deduced_params/one.carbon b/toolchain/parser/testdata/generics/deduced_params/one.carbon
new file mode 100644
index 000000000000..653efc807618
--- /dev/null
+++ b/toolchain/parser/testdata/generics/deduced_params/one.carbon
@@ -0,0 +1,34 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// RUN: %{carbon-run-parser}
+// CHECK:STDOUT: [
+// CHECK:STDOUT:   {kind: 'ClassIntroducer', text: 'class'},
+// CHECK:STDOUT:   {kind: 'DeclaredName', text: 'Foo'},
+// CHECK:STDOUT:     {kind: 'DeducedParameterListStart', text: '['},
+// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'a'},
+// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:   {kind: 'DeducedParameterList', text: ']', subtree_size: 5},
+// CHECK:STDOUT:     {kind: 'ParameterListStart', text: '('},
+// CHECK:STDOUT:   {kind: 'ParameterList', text: ')', subtree_size: 2},
+// CHECK:STDOUT: {kind: 'ClassDeclaration', text: ';', subtree_size: 10},
+// CHECK:STDOUT:     {kind: 'InterfaceIntroducer', text: 'interface'},
+// CHECK:STDOUT:     {kind: 'DeclaredName', text: 'Bar'},
+// CHECK:STDOUT:       {kind: 'DeducedParameterListStart', text: '['},
+// CHECK:STDOUT:         {kind: 'DeclaredName', text: 'a'},
+// CHECK:STDOUT:         {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:       {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:     {kind: 'DeducedParameterList', text: ']', subtree_size: 5},
+// CHECK:STDOUT:       {kind: 'ParameterListStart', text: '('},
+// CHECK:STDOUT:     {kind: 'ParameterList', text: ')', subtree_size: 2},
+// CHECK:STDOUT:   {kind: 'InterfaceDefinitionStart', text: '{', subtree_size: 10},
+// CHECK:STDOUT: {kind: 'InterfaceDefinition', text: '}', subtree_size: 11},
+// CHECK:STDOUT: {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT: ]
+
+class Foo[a: i32]();
+
+interface Bar[a: i32]() {}
diff --git a/toolchain/parser/testdata/generics/deduced_params/one_suffix_comma.carbon b/toolchain/parser/testdata/generics/deduced_params/one_suffix_comma.carbon
new file mode 100644
index 000000000000..5f4951cca627
--- /dev/null
+++ b/toolchain/parser/testdata/generics/deduced_params/one_suffix_comma.carbon
@@ -0,0 +1,36 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// RUN: %{carbon-run-parser}
+// CHECK:STDOUT: [
+// CHECK:STDOUT:   {kind: 'ClassIntroducer', text: 'class'},
+// CHECK:STDOUT:   {kind: 'DeclaredName', text: 'Foo'},
+// CHECK:STDOUT:     {kind: 'DeducedParameterListStart', text: '['},
+// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'a'},
+// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:   {kind: 'DeducedParameterList', text: ']', subtree_size: 6},
+// CHECK:STDOUT:     {kind: 'ParameterListStart', text: '('},
+// CHECK:STDOUT:   {kind: 'ParameterList', text: ')', subtree_size: 2},
+// CHECK:STDOUT: {kind: 'ClassDeclaration', text: ';', subtree_size: 11},
+// CHECK:STDOUT:     {kind: 'InterfaceIntroducer', text: 'interface'},
+// CHECK:STDOUT:     {kind: 'DeclaredName', text: 'Bar'},
+// CHECK:STDOUT:       {kind: 'DeducedParameterListStart', text: '['},
+// CHECK:STDOUT:         {kind: 'DeclaredName', text: 'a'},
+// CHECK:STDOUT:         {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:       {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:     {kind: 'DeducedParameterList', text: ']', subtree_size: 6},
+// CHECK:STDOUT:       {kind: 'ParameterListStart', text: '('},
+// CHECK:STDOUT:     {kind: 'ParameterList', text: ')', subtree_size: 2},
+// CHECK:STDOUT:   {kind: 'InterfaceDefinitionStart', text: '{', subtree_size: 11},
+// CHECK:STDOUT: {kind: 'InterfaceDefinition', text: '}', subtree_size: 12},
+// CHECK:STDOUT: {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT: ]
+
+class Foo[a: i32,]();
+
+interface Bar[a: i32,]() {}
diff --git a/toolchain/parser/testdata/generics/deduced_params/six.carbon b/toolchain/parser/testdata/generics/deduced_params/six.carbon
new file mode 100644
index 000000000000..0c555efc8000
--- /dev/null
+++ b/toolchain/parser/testdata/generics/deduced_params/six.carbon
@@ -0,0 +1,74 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// RUN: %{carbon-run-parser}
+// CHECK:STDOUT: [
+// CHECK:STDOUT:   {kind: 'ClassIntroducer', text: 'class'},
+// CHECK:STDOUT:   {kind: 'DeclaredName', text: 'Foo'},
+// CHECK:STDOUT:     {kind: 'DeducedParameterListStart', text: '['},
+// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'a'},
+// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'b'},
+// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'c'},
+// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'd'},
+// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'e'},
+// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'f'},
+// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:   {kind: 'DeducedParameterList', text: ']', subtree_size: 25},
+// CHECK:STDOUT:     {kind: 'ParameterListStart', text: '('},
+// CHECK:STDOUT:   {kind: 'ParameterList', text: ')', subtree_size: 2},
+// CHECK:STDOUT: {kind: 'ClassDeclaration', text: ';', subtree_size: 30},
+// CHECK:STDOUT:     {kind: 'InterfaceIntroducer', text: 'interface'},
+// CHECK:STDOUT:     {kind: 'DeclaredName', text: 'Bar'},
+// CHECK:STDOUT:       {kind: 'DeducedParameterListStart', text: '['},
+// CHECK:STDOUT:         {kind: 'DeclaredName', text: 'a'},
+// CHECK:STDOUT:         {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:       {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:         {kind: 'DeclaredName', text: 'b'},
+// CHECK:STDOUT:         {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:       {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:         {kind: 'DeclaredName', text: 'c'},
+// CHECK:STDOUT:         {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:       {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:         {kind: 'DeclaredName', text: 'd'},
+// CHECK:STDOUT:         {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:       {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:         {kind: 'DeclaredName', text: 'e'},
+// CHECK:STDOUT:         {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:       {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:         {kind: 'DeclaredName', text: 'f'},
+// CHECK:STDOUT:         {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:       {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:     {kind: 'DeducedParameterList', text: ']', subtree_size: 25},
+// CHECK:STDOUT:       {kind: 'ParameterListStart', text: '('},
+// CHECK:STDOUT:     {kind: 'ParameterList', text: ')', subtree_size: 2},
+// CHECK:STDOUT:   {kind: 'InterfaceDefinitionStart', text: '{', subtree_size: 30},
+// CHECK:STDOUT: {kind: 'InterfaceDefinition', text: '}', subtree_size: 31},
+// CHECK:STDOUT: {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT: ]
+
+class Foo[a: i32, b: i32, c: i32, d: i32, e: i32, f: i32]();
+
+interface Bar[a: i32, b: i32, c: i32, d: i32, e: i32, f: i32]() {}
diff --git a/toolchain/parser/testdata/generics/deduced_params/two.carbon b/toolchain/parser/testdata/generics/deduced_params/two.carbon
new file mode 100644
index 000000000000..beaf9b21343f
--- /dev/null
+++ b/toolchain/parser/testdata/generics/deduced_params/two.carbon
@@ -0,0 +1,42 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// RUN: %{carbon-run-parser}
+// CHECK:STDOUT: [
+// CHECK:STDOUT:   {kind: 'ClassIntroducer', text: 'class'},
+// CHECK:STDOUT:   {kind: 'DeclaredName', text: 'Foo'},
+// CHECK:STDOUT:     {kind: 'DeducedParameterListStart', text: '['},
+// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'a'},
+// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'b'},
+// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:   {kind: 'DeducedParameterList', text: ']', subtree_size: 9},
+// CHECK:STDOUT:     {kind: 'ParameterListStart', text: '('},
+// CHECK:STDOUT:   {kind: 'ParameterList', text: ')', subtree_size: 2},
+// CHECK:STDOUT: {kind: 'ClassDeclaration', text: ';', subtree_size: 14},
+// CHECK:STDOUT:     {kind: 'InterfaceIntroducer', text: 'interface'},
+// CHECK:STDOUT:     {kind: 'DeclaredName', text: 'Bar'},
+// CHECK:STDOUT:       {kind: 'DeducedParameterListStart', text: '['},
+// CHECK:STDOUT:         {kind: 'DeclaredName', text: 'a'},
+// CHECK:STDOUT:         {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:       {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:         {kind: 'DeclaredName', text: 'b'},
+// CHECK:STDOUT:         {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:       {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:     {kind: 'DeducedParameterList', text: ']', subtree_size: 9},
+// CHECK:STDOUT:       {kind: 'ParameterListStart', text: '('},
+// CHECK:STDOUT:     {kind: 'ParameterList', text: ')', subtree_size: 2},
+// CHECK:STDOUT:   {kind: 'InterfaceDefinitionStart', text: '{', subtree_size: 14},
+// CHECK:STDOUT: {kind: 'InterfaceDefinition', text: '}', subtree_size: 15},
+// CHECK:STDOUT: {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT: ]
+
+class Foo[a: i32, b: i32]();
+
+interface Bar[a: i32, b: i32]() {}
diff --git a/toolchain/parser/testdata/generics/deduced_params/two_suffix_comma.carbon b/toolchain/parser/testdata/generics/deduced_params/two_suffix_comma.carbon
new file mode 100644
index 000000000000..f70c0daf9648
--- /dev/null
+++ b/toolchain/parser/testdata/generics/deduced_params/two_suffix_comma.carbon
@@ -0,0 +1,44 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// RUN: %{carbon-run-parser}
+// CHECK:STDOUT: [
+// CHECK:STDOUT:   {kind: 'ClassIntroducer', text: 'class'},
+// CHECK:STDOUT:   {kind: 'DeclaredName', text: 'Foo'},
+// CHECK:STDOUT:     {kind: 'DeducedParameterListStart', text: '['},
+// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'a'},
+// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'b'},
+// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:   {kind: 'DeducedParameterList', text: ']', subtree_size: 10},
+// CHECK:STDOUT:     {kind: 'ParameterListStart', text: '('},
+// CHECK:STDOUT:   {kind: 'ParameterList', text: ')', subtree_size: 2},
+// CHECK:STDOUT: {kind: 'ClassDeclaration', text: ';', subtree_size: 15},
+// CHECK:STDOUT:     {kind: 'InterfaceIntroducer', text: 'interface'},
+// CHECK:STDOUT:     {kind: 'DeclaredName', text: 'Bar'},
+// CHECK:STDOUT:       {kind: 'DeducedParameterListStart', text: '['},
+// CHECK:STDOUT:         {kind: 'DeclaredName', text: 'a'},
+// CHECK:STDOUT:         {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:       {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:         {kind: 'DeclaredName', text: 'b'},
+// CHECK:STDOUT:         {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:       {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:     {kind: 'DeducedParameterList', text: ']', subtree_size: 10},
+// CHECK:STDOUT:       {kind: 'ParameterListStart', text: '('},
+// CHECK:STDOUT:     {kind: 'ParameterList', text: ')', subtree_size: 2},
+// CHECK:STDOUT:   {kind: 'InterfaceDefinitionStart', text: '{', subtree_size: 15},
+// CHECK:STDOUT: {kind: 'InterfaceDefinition', text: '}', subtree_size: 16},
+// CHECK:STDOUT: {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT: ]
+
+class Foo[a: i32, b: i32,]();
+
+interface Bar[a: i32, b: i32,]() {}
diff --git a/toolchain/parser/testdata/generics/params/empty.carbon b/toolchain/parser/testdata/generics/params/empty.carbon
new file mode 100644
index 000000000000..28fac6a361a2
--- /dev/null
+++ b/toolchain/parser/testdata/generics/params/empty.carbon
@@ -0,0 +1,24 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// RUN: %{carbon-run-parser}
+// CHECK:STDOUT: [
+// CHECK:STDOUT:   {kind: 'ClassIntroducer', text: 'class'},
+// CHECK:STDOUT:   {kind: 'DeclaredName', text: 'Foo'},
+// CHECK:STDOUT:     {kind: 'ParameterListStart', text: '('},
+// CHECK:STDOUT:   {kind: 'ParameterList', text: ')', subtree_size: 2},
+// CHECK:STDOUT: {kind: 'ClassDeclaration', text: ';', subtree_size: 5},
+// CHECK:STDOUT:     {kind: 'InterfaceIntroducer', text: 'interface'},
+// CHECK:STDOUT:     {kind: 'DeclaredName', text: 'Bar'},
+// CHECK:STDOUT:       {kind: 'ParameterListStart', text: '('},
+// CHECK:STDOUT:     {kind: 'ParameterList', text: ')', subtree_size: 2},
+// CHECK:STDOUT:   {kind: 'InterfaceDefinitionStart', text: '{', subtree_size: 5},
+// CHECK:STDOUT: {kind: 'InterfaceDefinition', text: '}', subtree_size: 6},
+// CHECK:STDOUT: {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT: ]
+
+class Foo();
+
+interface Bar() {}
diff --git a/toolchain/parser/testdata/generics/params/one.carbon b/toolchain/parser/testdata/generics/params/one.carbon
new file mode 100644
index 000000000000..df707855567f
--- /dev/null
+++ b/toolchain/parser/testdata/generics/params/one.carbon
@@ -0,0 +1,30 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// RUN: %{carbon-run-parser}
+// CHECK:STDOUT: [
+// CHECK:STDOUT:   {kind: 'ClassIntroducer', text: 'class'},
+// CHECK:STDOUT:   {kind: 'DeclaredName', text: 'Foo'},
+// CHECK:STDOUT:     {kind: 'ParameterListStart', text: '('},
+// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'a'},
+// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:   {kind: 'ParameterList', text: ')', subtree_size: 5},
+// CHECK:STDOUT: {kind: 'ClassDeclaration', text: ';', subtree_size: 8},
+// CHECK:STDOUT:     {kind: 'InterfaceIntroducer', text: 'interface'},
+// CHECK:STDOUT:     {kind: 'DeclaredName', text: 'Bar'},
+// CHECK:STDOUT:       {kind: 'ParameterListStart', text: '('},
+// CHECK:STDOUT:         {kind: 'DeclaredName', text: 'a'},
+// CHECK:STDOUT:         {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:       {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:     {kind: 'ParameterList', text: ')', subtree_size: 5},
+// CHECK:STDOUT:   {kind: 'InterfaceDefinitionStart', text: '{', subtree_size: 8},
+// CHECK:STDOUT: {kind: 'InterfaceDefinition', text: '}', subtree_size: 9},
+// CHECK:STDOUT: {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT: ]
+
+class Foo(a: i32);
+
+interface Bar(a: i32) {}
diff --git a/toolchain/parser/testdata/generics/params/one_suffix_comma.carbon b/toolchain/parser/testdata/generics/params/one_suffix_comma.carbon
new file mode 100644
index 000000000000..4673ea28bfde
--- /dev/null
+++ b/toolchain/parser/testdata/generics/params/one_suffix_comma.carbon
@@ -0,0 +1,32 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// RUN: %{carbon-run-parser}
+// CHECK:STDOUT: [
+// CHECK:STDOUT:   {kind: 'ClassIntroducer', text: 'class'},
+// CHECK:STDOUT:   {kind: 'DeclaredName', text: 'Foo'},
+// CHECK:STDOUT:     {kind: 'ParameterListStart', text: '('},
+// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'a'},
+// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:   {kind: 'ParameterList', text: ')', subtree_size: 6},
+// CHECK:STDOUT: {kind: 'ClassDeclaration', text: ';', subtree_size: 9},
+// CHECK:STDOUT:     {kind: 'InterfaceIntroducer', text: 'interface'},
+// CHECK:STDOUT:     {kind: 'DeclaredName', text: 'Bar'},
+// CHECK:STDOUT:       {kind: 'ParameterListStart', text: '('},
+// CHECK:STDOUT:         {kind: 'DeclaredName', text: 'a'},
+// CHECK:STDOUT:         {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:       {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:     {kind: 'ParameterList', text: ')', subtree_size: 6},
+// CHECK:STDOUT:   {kind: 'InterfaceDefinitionStart', text: '{', subtree_size: 9},
+// CHECK:STDOUT: {kind: 'InterfaceDefinition', text: '}', subtree_size: 10},
+// CHECK:STDOUT: {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT: ]
+
+class Foo(a: i32,);
+
+interface Bar(a: i32,) {}
diff --git a/toolchain/parser/testdata/generics/params/six.carbon b/toolchain/parser/testdata/generics/params/six.carbon
new file mode 100644
index 000000000000..e2741420cafc
--- /dev/null
+++ b/toolchain/parser/testdata/generics/params/six.carbon
@@ -0,0 +1,70 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// RUN: %{carbon-run-parser}
+// CHECK:STDOUT: [
+// CHECK:STDOUT:   {kind: 'ClassIntroducer', text: 'class'},
+// CHECK:STDOUT:   {kind: 'DeclaredName', text: 'Foo'},
+// CHECK:STDOUT:     {kind: 'ParameterListStart', text: '('},
+// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'a'},
+// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'b'},
+// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'c'},
+// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'd'},
+// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'e'},
+// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'f'},
+// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:   {kind: 'ParameterList', text: ')', subtree_size: 25},
+// CHECK:STDOUT: {kind: 'ClassDeclaration', text: ';', subtree_size: 28},
+// CHECK:STDOUT:     {kind: 'InterfaceIntroducer', text: 'interface'},
+// CHECK:STDOUT:     {kind: 'DeclaredName', text: 'Bar'},
+// CHECK:STDOUT:       {kind: 'ParameterListStart', text: '('},
+// CHECK:STDOUT:         {kind: 'DeclaredName', text: 'a'},
+// CHECK:STDOUT:         {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:       {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:         {kind: 'DeclaredName', text: 'b'},
+// CHECK:STDOUT:         {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:       {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:         {kind: 'DeclaredName', text: 'c'},
+// CHECK:STDOUT:         {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:       {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:         {kind: 'DeclaredName', text: 'd'},
+// CHECK:STDOUT:         {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:       {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:         {kind: 'DeclaredName', text: 'e'},
+// CHECK:STDOUT:         {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:       {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:         {kind: 'DeclaredName', text: 'f'},
+// CHECK:STDOUT:         {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:       {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:     {kind: 'ParameterList', text: ')', subtree_size: 25},
+// CHECK:STDOUT:   {kind: 'InterfaceDefinitionStart', text: '{', subtree_size: 28},
+// CHECK:STDOUT: {kind: 'InterfaceDefinition', text: '}', subtree_size: 29},
+// CHECK:STDOUT: {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT: ]
+
+class Foo(a: i32, b: i32, c: i32, d: i32, e: i32, f: i32);
+
+interface Bar(a: i32, b: i32, c: i32, d: i32, e: i32, f: i32) {}
diff --git a/toolchain/parser/testdata/generics/params/two.carbon b/toolchain/parser/testdata/generics/params/two.carbon
new file mode 100644
index 000000000000..35f020bf5f5c
--- /dev/null
+++ b/toolchain/parser/testdata/generics/params/two.carbon
@@ -0,0 +1,38 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// RUN: %{carbon-run-parser}
+// CHECK:STDOUT: [
+// CHECK:STDOUT:   {kind: 'ClassIntroducer', text: 'class'},
+// CHECK:STDOUT:   {kind: 'DeclaredName', text: 'Foo'},
+// CHECK:STDOUT:     {kind: 'ParameterListStart', text: '('},
+// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'a'},
+// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'b'},
+// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:   {kind: 'ParameterList', text: ')', subtree_size: 9},
+// CHECK:STDOUT: {kind: 'ClassDeclaration', text: ';', subtree_size: 12},
+// CHECK:STDOUT:     {kind: 'InterfaceIntroducer', text: 'interface'},
+// CHECK:STDOUT:     {kind: 'DeclaredName', text: 'Bar'},
+// CHECK:STDOUT:       {kind: 'ParameterListStart', text: '('},
+// CHECK:STDOUT:         {kind: 'DeclaredName', text: 'a'},
+// CHECK:STDOUT:         {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:       {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:         {kind: 'DeclaredName', text: 'b'},
+// CHECK:STDOUT:         {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:       {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:     {kind: 'ParameterList', text: ')', subtree_size: 9},
+// CHECK:STDOUT:   {kind: 'InterfaceDefinitionStart', text: '{', subtree_size: 12},
+// CHECK:STDOUT: {kind: 'InterfaceDefinition', text: '}', subtree_size: 13},
+// CHECK:STDOUT: {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT: ]
+
+class Foo(a: i32, b: i32);
+
+interface Bar(a: i32, b: i32) {}
diff --git a/toolchain/parser/testdata/generics/params/two_suffix_comma.carbon b/toolchain/parser/testdata/generics/params/two_suffix_comma.carbon
new file mode 100644
index 000000000000..db229bd65030
--- /dev/null
+++ b/toolchain/parser/testdata/generics/params/two_suffix_comma.carbon
@@ -0,0 +1,40 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// RUN: %{carbon-run-parser}
+// CHECK:STDOUT: [
+// CHECK:STDOUT:   {kind: 'ClassIntroducer', text: 'class'},
+// CHECK:STDOUT:   {kind: 'DeclaredName', text: 'Foo'},
+// CHECK:STDOUT:     {kind: 'ParameterListStart', text: '('},
+// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'a'},
+// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:       {kind: 'DeclaredName', text: 'b'},
+// CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:     {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:   {kind: 'ParameterList', text: ')', subtree_size: 10},
+// CHECK:STDOUT: {kind: 'ClassDeclaration', text: ';', subtree_size: 13},
+// CHECK:STDOUT:     {kind: 'InterfaceIntroducer', text: 'interface'},
+// CHECK:STDOUT:     {kind: 'DeclaredName', text: 'Bar'},
+// CHECK:STDOUT:       {kind: 'ParameterListStart', text: '('},
+// CHECK:STDOUT:         {kind: 'DeclaredName', text: 'a'},
+// CHECK:STDOUT:         {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:       {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:         {kind: 'DeclaredName', text: 'b'},
+// CHECK:STDOUT:         {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:       {kind: 'PatternBinding', text: ':', subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'ParameterListComma', text: ','},
+// CHECK:STDOUT:     {kind: 'ParameterList', text: ')', subtree_size: 10},
+// CHECK:STDOUT:   {kind: 'InterfaceDefinitionStart', text: '{', subtree_size: 13},
+// CHECK:STDOUT: {kind: 'InterfaceDefinition', text: '}', subtree_size: 14},
+// CHECK:STDOUT: {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT: ]
+
+class Foo(a: i32, b: i32,);
+
+interface Bar(a: i32, b: i32,) {}