diff --git a/toolchain/diagnostics/diagnostic_kind.def b/toolchain/diagnostics/diagnostic_kind.def
index 970bc36d3449..a9833de36b55 100644
--- a/toolchain/diagnostics/diagnostic_kind.def
+++ b/toolchain/diagnostics/diagnostic_kind.def
@@ -106,7 +106,6 @@ CARBON_DIAGNOSTIC_KIND(ExpectedDeclName)
 CARBON_DIAGNOSTIC_KIND(ExpectedDeclSemi)
 CARBON_DIAGNOSTIC_KIND(ExpectedDeclSemiOrDefinition)
 CARBON_DIAGNOSTIC_KIND(ExpectedInitializerAfterLet)
-CARBON_DIAGNOSTIC_KIND(MethodImplNotAllowed)
 CARBON_DIAGNOSTIC_KIND(ParamsRequiredAfterImplicit)
 CARBON_DIAGNOSTIC_KIND(ParamsRequiredByIntroducer)
 CARBON_DIAGNOSTIC_KIND(ExpectedAfterBase)
diff --git a/toolchain/parse/context.cpp b/toolchain/parse/context.cpp
index d20333948747..aabc91f05838 100644
--- a/toolchain/parse/context.cpp
+++ b/toolchain/parse/context.cpp
@@ -417,30 +417,6 @@ auto Context::ConsumeListToken(NodeKind comma_kind, Lex::TokenKind close_kind,
   }
 }
 
-auto Context::GetDeclContext() -> DeclContext {
-  // i == 0 is the file-level DeclScopeLoop. Additionally, i == 1 can be
-  // skipped because it will never be a DeclScopeLoop.
-  for (int i = state_stack_.size() - 1; i > 1; --i) {
-    // The declaration context is always the state _above_ a
-    // DeclScopeLoop.
-    if (state_stack_[i].state == State::DeclScopeLoop) {
-      switch (state_stack_[i - 1].state) {
-        case State::TypeDefinitionFinishAsClass:
-          return DeclContext::Class;
-        case State::TypeDefinitionFinishAsInterface:
-          return DeclContext::Interface;
-        case State::TypeDefinitionFinishAsNamedConstraint:
-          return DeclContext::NamedConstraint;
-        default:
-          llvm_unreachable("Missing handling for a declaration scope");
-      }
-    }
-  }
-  CARBON_CHECK(!state_stack_.empty() &&
-               state_stack_[0].state == State::DeclScopeLoop);
-  return DeclContext::File;
-}
-
 auto Context::RecoverFromDeclError(StateStackEntry state,
                                    NodeKind parse_node_kind,
                                    bool skip_past_likely_end) -> void {
diff --git a/toolchain/parse/context.h b/toolchain/parse/context.h
index 2980c0474bbc..719accd91bd4 100644
--- a/toolchain/parse/context.h
+++ b/toolchain/parse/context.h
@@ -47,14 +47,6 @@ class Context {
     Let
   };
 
-  // Supported return values for GetDeclContext.
-  enum class DeclContext : int8_t {
-    File,  // Top-level context.
-    Class,
-    Interface,
-    NamedConstraint,
-  };
-
   // Used for restricting ordering of `package` and `import` directives.
   enum class PackagingState : int8_t {
     FileStart,
@@ -300,15 +292,6 @@ class Context {
         << "Excessive stack size: likely infinite loop";
   }
 
-  // Returns the current declaration context according to state_stack_.
-  // This is expected to be called in cases which are close to a context.
-  // Although it looks like it could be O(n) for state_stack_'s depth, valid
-  // parses should only need to look down a couple steps.
-  //
-  // This currently assumes it's being called from within the declaration's
-  // DeclScopeLoop.
-  auto GetDeclContext() -> DeclContext;
-
   // Propagates an error up the state stack, to the parent state.
   auto ReturnErrorOnState() -> void { state_stack_.back().has_error = true; }
 
diff --git a/toolchain/parse/handle_function.cpp b/toolchain/parse/handle_function.cpp
index 3b7cbc5db982..9e3738b0a263 100644
--- a/toolchain/parse/handle_function.cpp
+++ b/toolchain/parse/handle_function.cpp
@@ -46,18 +46,6 @@ auto HandleFunctionSignatureFinish(Context& context) -> void {
       break;
     }
     case Lex::TokenKind::OpenCurlyBrace: {
-      if (auto decl_context = context.GetDeclContext();
-          decl_context == Context::DeclContext::Interface ||
-          decl_context == Context::DeclContext::NamedConstraint) {
-        CARBON_DIAGNOSTIC(
-            MethodImplNotAllowed, Error,
-            "Method implementations are not allowed in interfaces.");
-        context.emitter().Emit(*context.position(), MethodImplNotAllowed);
-        context.RecoverFromDeclError(state, NodeKind::FunctionDecl,
-                                     /*skip_past_likely_end=*/true);
-        break;
-      }
-
       context.AddNode(NodeKind::FunctionDefinitionStart, context.Consume(),
                       state.subtree_start, state.has_error);
       // Any error is recorded on the FunctionDefinitionStart.
diff --git a/toolchain/parse/testdata/generics/interface/fail_no_impl_allowed.carbon b/toolchain/parse/testdata/generics/interface/fail_no_impl_allowed.carbon
deleted file mode 100644
index 59958c25487c..000000000000
--- a/toolchain/parse/testdata/generics/interface/fail_no_impl_allowed.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
-
-interface Foo {
-  // CHECK:STDERR: fail_no_impl_allowed.carbon:[[@LINE+3]]:39: ERROR: Method implementations are not allowed in interfaces.
-  // CHECK:STDERR:   fn Add[self: Self](b: Self) -> Self {
-  // CHECK:STDERR:                                       ^
-  fn Add[self: Self](b: Self) -> Self {
-    print("You can't do that.");
-  }
-}
-
-// CHECK:STDOUT: - filename: fail_no_impl_allowed.carbon
-// CHECK:STDOUT:   parse_tree: [
-// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
-// CHECK:STDOUT:         {kind: 'InterfaceIntroducer', text: 'interface'},
-// CHECK:STDOUT:         {kind: 'IdentifierName', text: 'Foo'},
-// CHECK:STDOUT:       {kind: 'InterfaceDefinitionStart', text: '{', subtree_size: 3},
-// CHECK:STDOUT:         {kind: 'FunctionIntroducer', text: 'fn'},
-// CHECK:STDOUT:         {kind: 'IdentifierName', text: 'Add'},
-// CHECK:STDOUT:           {kind: 'ImplicitParamListStart', text: '['},
-// CHECK:STDOUT:             {kind: 'SelfValueName', text: 'self'},
-// CHECK:STDOUT:             {kind: 'SelfTypeNameExpr', text: 'Self'},
-// CHECK:STDOUT:           {kind: 'BindingPattern', text: ':', subtree_size: 3},
-// CHECK:STDOUT:         {kind: 'ImplicitParamList', text: ']', subtree_size: 5},
-// CHECK:STDOUT:           {kind: 'ParamListStart', text: '('},
-// CHECK:STDOUT:             {kind: 'IdentifierName', text: 'b'},
-// CHECK:STDOUT:             {kind: 'SelfTypeNameExpr', text: 'Self'},
-// CHECK:STDOUT:           {kind: 'BindingPattern', text: ':', subtree_size: 3},
-// CHECK:STDOUT:         {kind: 'ParamList', text: ')', subtree_size: 5},
-// CHECK:STDOUT:           {kind: 'SelfTypeNameExpr', text: 'Self'},
-// CHECK:STDOUT:         {kind: 'ReturnType', text: '->', subtree_size: 2},
-// CHECK:STDOUT:       {kind: 'FunctionDecl', text: 'fn', has_error: yes, subtree_size: 15},
-// CHECK:STDOUT:     {kind: 'InterfaceDefinition', text: '}', subtree_size: 19},
-// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
-// CHECK:STDOUT:   ]
diff --git a/toolchain/parse/testdata/generics/interface/final_member_definition.carbon b/toolchain/parse/testdata/generics/interface/final_member_definition.carbon
new file mode 100644
index 000000000000..ffb6754b6d32
--- /dev/null
+++ b/toolchain/parse/testdata/generics/interface/final_member_definition.carbon
@@ -0,0 +1,43 @@
+// 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
+
+interface Foo {
+  final fn Add[self: Self](b: Self) -> Self {
+    print("Allowed.");
+  }
+}
+
+// CHECK:STDOUT: - filename: final_member_definition.carbon
+// CHECK:STDOUT:   parse_tree: [
+// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
+// CHECK:STDOUT:         {kind: 'InterfaceIntroducer', text: 'interface'},
+// CHECK:STDOUT:         {kind: 'IdentifierName', text: 'Foo'},
+// CHECK:STDOUT:       {kind: 'InterfaceDefinitionStart', text: '{', subtree_size: 3},
+// CHECK:STDOUT:           {kind: 'FunctionIntroducer', text: 'fn'},
+// CHECK:STDOUT:           {kind: 'FinalModifier', text: 'final'},
+// CHECK:STDOUT:           {kind: 'IdentifierName', text: 'Add'},
+// CHECK:STDOUT:             {kind: 'ImplicitParamListStart', text: '['},
+// CHECK:STDOUT:               {kind: 'SelfValueName', text: 'self'},
+// CHECK:STDOUT:               {kind: 'SelfTypeNameExpr', text: 'Self'},
+// CHECK:STDOUT:             {kind: 'BindingPattern', text: ':', subtree_size: 3},
+// CHECK:STDOUT:           {kind: 'ImplicitParamList', text: ']', subtree_size: 5},
+// CHECK:STDOUT:             {kind: 'ParamListStart', text: '('},
+// CHECK:STDOUT:               {kind: 'IdentifierName', text: 'b'},
+// CHECK:STDOUT:               {kind: 'SelfTypeNameExpr', text: 'Self'},
+// CHECK:STDOUT:             {kind: 'BindingPattern', text: ':', subtree_size: 3},
+// CHECK:STDOUT:           {kind: 'ParamList', text: ')', subtree_size: 5},
+// CHECK:STDOUT:             {kind: 'SelfTypeNameExpr', text: 'Self'},
+// CHECK:STDOUT:           {kind: 'ReturnType', text: '->', subtree_size: 2},
+// CHECK:STDOUT:         {kind: 'FunctionDefinitionStart', text: '{', subtree_size: 16},
+// CHECK:STDOUT:               {kind: 'IdentifierNameExpr', text: 'print'},
+// CHECK:STDOUT:             {kind: 'CallExprStart', text: '(', subtree_size: 2},
+// CHECK:STDOUT:             {kind: 'StringLiteral', text: '"Allowed."'},
+// CHECK:STDOUT:           {kind: 'CallExpr', text: ')', subtree_size: 4},
+// CHECK:STDOUT:         {kind: 'ExprStatement', text: ';', subtree_size: 5},
+// CHECK:STDOUT:       {kind: 'FunctionDefinition', text: '}', subtree_size: 22},
+// CHECK:STDOUT:     {kind: 'InterfaceDefinition', text: '}', subtree_size: 26},
+// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT:   ]
diff --git a/toolchain/parse/testdata/generics/named_constraint/defined_method.carbon b/toolchain/parse/testdata/generics/named_constraint/defined_method.carbon
new file mode 100644
index 000000000000..2aa6e115f0f6
--- /dev/null
+++ b/toolchain/parse/testdata/generics/named_constraint/defined_method.carbon
@@ -0,0 +1,35 @@
+// 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
+
+constraint Foo {
+  fn Add[self: Self](b: Self) -> Self {}
+}
+
+// CHECK:STDOUT: - filename: defined_method.carbon
+// CHECK:STDOUT:   parse_tree: [
+// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
+// CHECK:STDOUT:         {kind: 'NamedConstraintIntroducer', text: 'constraint'},
+// CHECK:STDOUT:         {kind: 'IdentifierName', text: 'Foo'},
+// CHECK:STDOUT:       {kind: 'NamedConstraintDefinitionStart', text: '{', subtree_size: 3},
+// CHECK:STDOUT:           {kind: 'FunctionIntroducer', text: 'fn'},
+// CHECK:STDOUT:           {kind: 'IdentifierName', text: 'Add'},
+// CHECK:STDOUT:             {kind: 'ImplicitParamListStart', text: '['},
+// CHECK:STDOUT:               {kind: 'SelfValueName', text: 'self'},
+// CHECK:STDOUT:               {kind: 'SelfTypeNameExpr', text: 'Self'},
+// CHECK:STDOUT:             {kind: 'BindingPattern', text: ':', subtree_size: 3},
+// CHECK:STDOUT:           {kind: 'ImplicitParamList', text: ']', subtree_size: 5},
+// CHECK:STDOUT:             {kind: 'ParamListStart', text: '('},
+// CHECK:STDOUT:               {kind: 'IdentifierName', text: 'b'},
+// CHECK:STDOUT:               {kind: 'SelfTypeNameExpr', text: 'Self'},
+// CHECK:STDOUT:             {kind: 'BindingPattern', text: ':', subtree_size: 3},
+// CHECK:STDOUT:           {kind: 'ParamList', text: ')', subtree_size: 5},
+// CHECK:STDOUT:             {kind: 'SelfTypeNameExpr', text: 'Self'},
+// CHECK:STDOUT:           {kind: 'ReturnType', text: '->', subtree_size: 2},
+// CHECK:STDOUT:         {kind: 'FunctionDefinitionStart', text: '{', subtree_size: 15},
+// CHECK:STDOUT:       {kind: 'FunctionDefinition', text: '}', subtree_size: 16},
+// CHECK:STDOUT:     {kind: 'NamedConstraintDefinition', text: '}', subtree_size: 20},
+// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT:   ]
diff --git a/toolchain/parse/testdata/generics/named_constraint/fail_no_impl_allowed.carbon b/toolchain/parse/testdata/generics/named_constraint/fail_no_impl_allowed.carbon
deleted file mode 100644
index 6796d6e0afce..000000000000
--- a/toolchain/parse/testdata/generics/named_constraint/fail_no_impl_allowed.carbon
+++ /dev/null
@@ -1,37 +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
-
-constraint Foo {
-  // CHECK:STDERR: fail_no_impl_allowed.carbon:[[@LINE+3]]:39: ERROR: Method implementations are not allowed in interfaces.
-  // CHECK:STDERR:   fn Add[self: Self](b: Self) -> Self {}
-  // CHECK:STDERR:                                       ^
-  fn Add[self: Self](b: Self) -> Self {}
-}
-
-// CHECK:STDOUT: - filename: fail_no_impl_allowed.carbon
-// CHECK:STDOUT:   parse_tree: [
-// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
-// CHECK:STDOUT:         {kind: 'NamedConstraintIntroducer', text: 'constraint'},
-// CHECK:STDOUT:         {kind: 'IdentifierName', text: 'Foo'},
-// CHECK:STDOUT:       {kind: 'NamedConstraintDefinitionStart', text: '{', subtree_size: 3},
-// CHECK:STDOUT:         {kind: 'FunctionIntroducer', text: 'fn'},
-// CHECK:STDOUT:         {kind: 'IdentifierName', text: 'Add'},
-// CHECK:STDOUT:           {kind: 'ImplicitParamListStart', text: '['},
-// CHECK:STDOUT:             {kind: 'SelfValueName', text: 'self'},
-// CHECK:STDOUT:             {kind: 'SelfTypeNameExpr', text: 'Self'},
-// CHECK:STDOUT:           {kind: 'BindingPattern', text: ':', subtree_size: 3},
-// CHECK:STDOUT:         {kind: 'ImplicitParamList', text: ']', subtree_size: 5},
-// CHECK:STDOUT:           {kind: 'ParamListStart', text: '('},
-// CHECK:STDOUT:             {kind: 'IdentifierName', text: 'b'},
-// CHECK:STDOUT:             {kind: 'SelfTypeNameExpr', text: 'Self'},
-// CHECK:STDOUT:           {kind: 'BindingPattern', text: ':', subtree_size: 3},
-// CHECK:STDOUT:         {kind: 'ParamList', text: ')', subtree_size: 5},
-// CHECK:STDOUT:           {kind: 'SelfTypeNameExpr', text: 'Self'},
-// CHECK:STDOUT:         {kind: 'ReturnType', text: '->', subtree_size: 2},
-// CHECK:STDOUT:       {kind: 'FunctionDecl', text: 'fn', has_error: yes, subtree_size: 15},
-// CHECK:STDOUT:     {kind: 'NamedConstraintDefinition', text: '}', subtree_size: 19},
-// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
-// CHECK:STDOUT:   ]