Lex and parse support for ... statements and expand. (#7832)

No support for `each` yet, but this should be enough that we can try out
an implementation approach supporting expanding a statement across
elements of a tuple without building "real" variadics.

Assisted-by: Claude via Antigravity

toolchain/check/handle_pack_expansion.cpp

@@ -0,0 +1,25 @@
+// 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
+
+#include "toolchain/check/context.h"
+#include "toolchain/check/handle.h"
+
+namespace Carbon::Check {
+
+auto HandleParseNode(Context& context, Parse::PackExpansionStartId node_id)
+    -> bool {
+  return context.TODO(node_id, "HandlePackExpansionStart");
+}
+
+auto HandleParseNode(Context& context, Parse::PackExpansionStatementId node_id)
+    -> bool {
+  return context.TODO(node_id, "HandlePackExpansionStatement");
+}
+
+auto HandleParseNode(Context& context, Parse::PrefixOperatorExpandId node_id)
+    -> bool {
+  return context.TODO(node_id, "HandlePrefixOperatorExpand");
+}
+
+}  // namespace Carbon::Check

toolchain/check/node_stack.h

@@ -468,6 +468,7 @@ class NodeStack {
       case Parse::NodeKind::LetInitializer:
       case Parse::NodeKind::LetIntroducer:
       case Parse::NodeKind::NamedConstraintIntroducer:
+      case Parse::NodeKind::PackExpansionStart:
       case Parse::NodeKind::RefBindingName:
       case Parse::NodeKind::RuntimeBindingName:
       case Parse::NodeKind::ReturnStatementStart:

toolchain/check/testdata/variadics/pack_expansion.carbon

@@ -0,0 +1,33 @@
+// 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
+//
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/primitives.carbon
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/variadics/pack_expansion.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/variadics/pack_expansion.carbon
+
+// --- fail_todo_pack_expansion_statement.carbon
+library "[[@TEST_NAME]]";
+
+fn F() {
+  // CHECK:STDERR: fail_todo_pack_expansion_statement.carbon:[[@LINE+4]]:3: error: semantics TODO: `HandlePackExpansionStart` [SemanticsTodo]
+  // CHECK:STDERR:   ... F();
+  // CHECK:STDERR:   ^~~
+  // CHECK:STDERR:
+  ... F();
+}
+
+// --- fail_todo_expand.carbon
+library "[[@TEST_NAME]]";
+
+fn F(x: (i32, i32)) {
+  // CHECK:STDERR: fail_todo_expand.carbon:[[@LINE+4]]:3: error: semantics TODO: `HandlePrefixOperatorExpand` [SemanticsTodo]
+  // CHECK:STDERR:   expand x;
+  // CHECK:STDERR:   ^~~~~~~~
+  // CHECK:STDERR:
+  expand x;
+}

toolchain/lex/testdata/periods.carbon

@@ -0,0 +1,23 @@
+// 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
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/lex/testdata/periods.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/lex/testdata/periods.carbon
+// CHECK:STDOUT: - filename: periods.carbon
+// CHECK:STDOUT:   tokens:
+
+. .. ... .... ...... ...expand
+// CHECK:STDOUT:   - { index:  1, kind:             "Period", line: {{ *}}[[@LINE-1]], column:   1, indent: 1, spelling: ".", has_leading_space: true }
+// CHECK:STDOUT:   - { index:  2, kind:             "Period", line: {{ *}}[[@LINE-2]], column:   3, indent: 1, spelling: ".", has_leading_space: true }
+// CHECK:STDOUT:   - { index:  3, kind:             "Period", line: {{ *}}[[@LINE-3]], column:   4, indent: 1, spelling: "." }
+// CHECK:STDOUT:   - { index:  4, kind: "PeriodPeriodPeriod", line: {{ *}}[[@LINE-4]], column:   6, indent: 1, spelling: "...", has_leading_space: true }
+// CHECK:STDOUT:   - { index:  5, kind: "PeriodPeriodPeriod", line: {{ *}}[[@LINE-5]], column:  10, indent: 1, spelling: "...", has_leading_space: true }
+// CHECK:STDOUT:   - { index:  6, kind:             "Period", line: {{ *}}[[@LINE-6]], column:  13, indent: 1, spelling: "." }
+// CHECK:STDOUT:   - { index:  7, kind: "PeriodPeriodPeriod", line: {{ *}}[[@LINE-7]], column:  15, indent: 1, spelling: "...", has_leading_space: true }
+// CHECK:STDOUT:   - { index:  8, kind: "PeriodPeriodPeriod", line: {{ *}}[[@LINE-8]], column:  18, indent: 1, spelling: "..." }
+// CHECK:STDOUT:   - { index:  9, kind: "PeriodPeriodPeriod", line: {{ *}}[[@LINE-9]], column:  22, indent: 1, spelling: "...", has_leading_space: true }
+// CHECK:STDOUT:   - { index: 10, kind:             "Expand", line: {{ *}}[[@LINE-10]], column:  25, indent: 1, spelling: "expand" }

toolchain/lex/token_kind.def

@@ -60,6 +60,8 @@ CARBON_SYMBOL_TOKEN(GreaterGreaterEqual, ">>=")
 CARBON_SYMBOL_TOKEN(LessEqualGreater,    "<=>")
 CARBON_SYMBOL_TOKEN(LessLessEqual,       "<<=")
 CARBON_SYMBOL_TOKEN(MinusGreaterQuestion,"->?")
+CARBON_TOKEN_WITH_VIRTUAL_NODE(
+  CARBON_SYMBOL_TOKEN(PeriodPeriodPeriod,  "..."))
 
 CARBON_SYMBOL_TOKEN(AmpEqual,            "&=")
 CARBON_SYMBOL_TOKEN(CaretEqual,          "^=")
@@ -189,6 +191,7 @@ CARBON_KEYWORD_TOKEN(Cpp,                 "Cpp")
 CARBON_KEYWORD_TOKEN(Default,             "default")
 CARBON_KEYWORD_TOKEN(Else,                "else")
 CARBON_KEYWORD_TOKEN(Eval,                "eval")
+CARBON_KEYWORD_TOKEN(Expand,              "expand")
 CARBON_KEYWORD_TOKEN(Extend,              "extend")
 CARBON_KEYWORD_TOKEN(Extern,              "extern")
 CARBON_KEYWORD_TOKEN(False,               "false")

toolchain/parse/handle_statement.cpp

@@ -51,6 +51,12 @@ auto HandleStatement(Context& context) -> void {
       context.PushState(StateKind::MatchIntroducer);
       break;
     }
+    case Lex::TokenKind::PeriodPeriodPeriod: {
+      context.PushState(StateKind::StatementPackExpansionFinish);
+      context.AddLeafNode(NodeKind::PackExpansionStart, context.Consume());
+      context.PushState(StateKind::Statement);
+      break;
+    }
 #define CARBON_PARSE_NODE_KIND(Name)
 #define CARBON_PARSE_NODE_KIND_TOKEN_MODIFIER(Name) case Lex::TokenKind::Name:
 #include "toolchain/parse/node_kind.def"
@@ -228,6 +234,13 @@ auto HandleStatementReturnFinish(Context& context) -> void {
   HandleStatementKeywordFinish(context, NodeKind::ReturnStatement);
 }
 
+auto HandleStatementPackExpansionFinish(Context& context) -> void {
+  auto state = context.PopState();
+
+  context.AddNode(NodeKind::PackExpansionStatement, state.token,
+                  state.has_error);
+}
+
 auto HandleStatementScopeLoop(Context& context) -> void {
   // This maintains the current state until we're at the end of the scope.
 

toolchain/parse/node_kind.def

@@ -286,6 +286,9 @@ CARBON_PARSE_NODE_KIND_STATEMENT(WhileConditionStart)
 CARBON_PARSE_NODE_KIND_STATEMENT(WhileCondition)
 CARBON_PARSE_NODE_KIND_STATEMENT(WhileStatement)
 
+CARBON_PARSE_NODE_KIND_STATEMENT(PackExpansionStart)
+CARBON_PARSE_NODE_KIND_STATEMENT(PackExpansionStatement)
+
 CARBON_PARSE_NODE_KIND_EXPRESSION(IndexExprStart)
 CARBON_PARSE_NODE_KIND_EXPRESSION(IndexExpr)
 
@@ -337,6 +340,7 @@ CARBON_PARSE_NODE_KIND_TOKEN_LITERAL(UnsignedIntTypeLiteral,
 CARBON_PARSE_NODE_KIND_PREFIX_OPERATOR(Amp)
 CARBON_PARSE_NODE_KIND_PREFIX_OPERATOR(Caret)
 CARBON_PARSE_NODE_KIND_PREFIX_OPERATOR(Const)
+CARBON_PARSE_NODE_KIND_PREFIX_OPERATOR(Expand)
 CARBON_PARSE_NODE_KIND_PREFIX_OPERATOR(Not)
 CARBON_PARSE_NODE_KIND_PREFIX_OPERATOR(Minus)
 CARBON_PARSE_NODE_KIND_PREFIX_OPERATOR(MinusMinus)

toolchain/parse/precedence.cpp

@@ -149,6 +149,7 @@ auto PrecedenceGroup::ForLeading(Lex::TokenKind kind)
   switch (kind) {
     case Lex::TokenKind::Star:
     case Lex::TokenKind::Amp:
+    case Lex::TokenKind::Expand:
       return PrecedenceGroup(TermPrefix);
 
     case Lex::TokenKind::Not:
@@ -249,6 +250,7 @@ auto PrecedenceGroup::ForTrailing(Lex::TokenKind kind, bool infix)
 
     // Prefix-only operators.
     case Lex::TokenKind::Const:
+    case Lex::TokenKind::Expand:
     case Lex::TokenKind::MinusMinus:
     case Lex::TokenKind::Not:
     case Lex::TokenKind::Partial:

toolchain/parse/testdata/variadics/expand.carbon

@@ -0,0 +1,196 @@
+// 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
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/parse/testdata/variadics/expand.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/parse/testdata/variadics/expand.carbon
+
+// --- basic.carbon
+
+var a: auto = expand x;
+
+// --- precedence.carbon
+
+// `expand` has the same precedence as prefix `*`.
+var a: auto = expand x.y;
+var b: auto = expand x.(0);
+var c: auto = expand F();
+var d: auto = expand x + 1;
+var e: auto = expand x * 2;
+var f: auto = expand *x;
+var g: auto = *expand x;
+var h: auto = expand expand x;
+var i: auto = -expand x;
+
+// --- fail_under_not.carbon
+
+// Like prefix `*`, `expand` requires parentheses under `not`.
+// CHECK:STDERR: fail_under_not.carbon:[[@LINE+4]]:19: error: parentheses are required around this unary `expand` operator [UnaryOperatorRequiresParentheses]
+// CHECK:STDERR: var a: auto = not expand x;
+// CHECK:STDERR:                   ^~~~~~
+// CHECK:STDERR:
+var a: auto = not expand x;
+
+// --- fail_as_postfix.carbon
+
+// CHECK:STDERR: fail_as_postfix.carbon:[[@LINE+4]]:17: error: `var` declarations must end with a `;` [ExpectedDeclSemi]
+// CHECK:STDERR: var a: auto = x expand;
+// CHECK:STDERR:                 ^~~~~~
+// CHECK:STDERR:
+var a: auto = x expand;
+
+// CHECK:STDOUT: - filename: basic.carbon
+// CHECK:STDOUT:   ╭─FileStart ''
+// CHECK:STDOUT:   │ ╭─VariableIntroducer 'var'
+// CHECK:STDOUT:   │ │   ╭─IdentifierNameNotBeforeSignature 'a'
+// CHECK:STDOUT:   │ │   ├─BindingPatternTypeStart ':'
+// CHECK:STDOUT:   │ │   ├─AutoTypeLiteral 'auto'
+// CHECK:STDOUT:   │ │ ╭─VarBindingPattern ':'
+// CHECK:STDOUT:   │ ├─VariablePattern 'var'
+// CHECK:STDOUT:   │ ├─VariableInitializer '='
+// CHECK:STDOUT:   │ │ ╭─IdentifierNameExpr 'x'
+// CHECK:STDOUT:   │ ├─PrefixOperatorExpand 'expand'
+// CHECK:STDOUT:   ├─VariableDecl ';'
+// CHECK:STDOUT:   ├─FileEnd ''
+// CHECK:STDOUT:   (root)
+// CHECK:STDOUT: - filename: precedence.carbon
+// CHECK:STDOUT:   ╭─FileStart ''
+// CHECK:STDOUT:   │ ╭─VariableIntroducer 'var'
+// CHECK:STDOUT:   │ │   ╭─IdentifierNameNotBeforeSignature 'a'
+// CHECK:STDOUT:   │ │   ├─BindingPatternTypeStart ':'
+// CHECK:STDOUT:   │ │   ├─AutoTypeLiteral 'auto'
+// CHECK:STDOUT:   │ │ ╭─VarBindingPattern ':'
+// CHECK:STDOUT:   │ ├─VariablePattern 'var'
+// CHECK:STDOUT:   │ ├─VariableInitializer '='
+// CHECK:STDOUT:   │ │   ╭─IdentifierNameExpr 'x'
+// CHECK:STDOUT:   │ │   ├─IdentifierNameNotBeforeSignature 'y'
+// CHECK:STDOUT:   │ │ ╭─MemberAccessExpr '.'
+// CHECK:STDOUT:   │ ├─PrefixOperatorExpand 'expand'
+// CHECK:STDOUT:   ├─VariableDecl ';'
+// CHECK:STDOUT:   │ ╭─VariableIntroducer 'var'
+// CHECK:STDOUT:   │ │   ╭─IdentifierNameNotBeforeSignature 'b'
+// CHECK:STDOUT:   │ │   ├─BindingPatternTypeStart ':'
+// CHECK:STDOUT:   │ │   ├─AutoTypeLiteral 'auto'
+// CHECK:STDOUT:   │ │ ╭─VarBindingPattern ':'
+// CHECK:STDOUT:   │ ├─VariablePattern 'var'
+// CHECK:STDOUT:   │ ├─VariableInitializer '='
+// CHECK:STDOUT:   │ │   ╭─IdentifierNameExpr 'x'
+// CHECK:STDOUT:   │ │   │ ╭─ParenExprStart '('
+// CHECK:STDOUT:   │ │   │ ├─IntLiteral '0'
+// CHECK:STDOUT:   │ │   ├─ParenExpr ')'
+// CHECK:STDOUT:   │ │ ╭─MemberAccessExpr '.'
+// CHECK:STDOUT:   │ ├─PrefixOperatorExpand 'expand'
+// CHECK:STDOUT:   ├─VariableDecl ';'
+// CHECK:STDOUT:   │ ╭─VariableIntroducer 'var'
+// CHECK:STDOUT:   │ │   ╭─IdentifierNameNotBeforeSignature 'c'
+// CHECK:STDOUT:   │ │   ├─BindingPatternTypeStart ':'
+// CHECK:STDOUT:   │ │   ├─AutoTypeLiteral 'auto'
+// CHECK:STDOUT:   │ │ ╭─VarBindingPattern ':'
+// CHECK:STDOUT:   │ ├─VariablePattern 'var'
+// CHECK:STDOUT:   │ ├─VariableInitializer '='
+// CHECK:STDOUT:   │ │     ╭─IdentifierNameExpr 'F'
+// CHECK:STDOUT:   │ │   ╭─CallExprStart '('
+// CHECK:STDOUT:   │ │ ╭─CallExpr ')'
+// CHECK:STDOUT:   │ ├─PrefixOperatorExpand 'expand'
+// CHECK:STDOUT:   ├─VariableDecl ';'
+// CHECK:STDOUT:   │ ╭─VariableIntroducer 'var'
+// CHECK:STDOUT:   │ │   ╭─IdentifierNameNotBeforeSignature 'd'
+// CHECK:STDOUT:   │ │   ├─BindingPatternTypeStart ':'
+// CHECK:STDOUT:   │ │   ├─AutoTypeLiteral 'auto'
+// CHECK:STDOUT:   │ │ ╭─VarBindingPattern ':'
+// CHECK:STDOUT:   │ ├─VariablePattern 'var'
+// CHECK:STDOUT:   │ ├─VariableInitializer '='
+// CHECK:STDOUT:   │ │   ╭─IdentifierNameExpr 'x'
+// CHECK:STDOUT:   │ │ ╭─PrefixOperatorExpand 'expand'
+// CHECK:STDOUT:   │ │ ├─IntLiteral '1'
+// CHECK:STDOUT:   │ ├─InfixOperatorPlus '+'
+// CHECK:STDOUT:   ├─VariableDecl ';'
+// CHECK:STDOUT:   │ ╭─VariableIntroducer 'var'
+// CHECK:STDOUT:   │ │   ╭─IdentifierNameNotBeforeSignature 'e'
+// CHECK:STDOUT:   │ │   ├─BindingPatternTypeStart ':'
+// CHECK:STDOUT:   │ │   ├─AutoTypeLiteral 'auto'
+// CHECK:STDOUT:   │ │ ╭─VarBindingPattern ':'
+// CHECK:STDOUT:   │ ├─VariablePattern 'var'
+// CHECK:STDOUT:   │ ├─VariableInitializer '='
+// CHECK:STDOUT:   │ │   ╭─IdentifierNameExpr 'x'
+// CHECK:STDOUT:   │ │ ╭─PrefixOperatorExpand 'expand'
+// CHECK:STDOUT:   │ │ ├─IntLiteral '2'
+// CHECK:STDOUT:   │ ├─InfixOperatorStar '*'
+// CHECK:STDOUT:   ├─VariableDecl ';'
+// CHECK:STDOUT:   │ ╭─VariableIntroducer 'var'
+// CHECK:STDOUT:   │ │   ╭─IdentifierNameNotBeforeSignature 'f'
+// CHECK:STDOUT:   │ │   ├─BindingPatternTypeStart ':'
+// CHECK:STDOUT:   │ │   ├─AutoTypeLiteral 'auto'
+// CHECK:STDOUT:   │ │ ╭─VarBindingPattern ':'
+// CHECK:STDOUT:   │ ├─VariablePattern 'var'
+// CHECK:STDOUT:   │ ├─VariableInitializer '='
+// CHECK:STDOUT:   │ │   ╭─IdentifierNameExpr 'x'
+// CHECK:STDOUT:   │ │ ╭─PrefixOperatorStar '*'
+// CHECK:STDOUT:   │ ├─PrefixOperatorExpand 'expand'
+// CHECK:STDOUT:   ├─VariableDecl ';'
+// CHECK:STDOUT:   │ ╭─VariableIntroducer 'var'
+// CHECK:STDOUT:   │ │   ╭─IdentifierNameNotBeforeSignature 'g'
+// CHECK:STDOUT:   │ │   ├─BindingPatternTypeStart ':'
+// CHECK:STDOUT:   │ │   ├─AutoTypeLiteral 'auto'
+// CHECK:STDOUT:   │ │ ╭─VarBindingPattern ':'
+// CHECK:STDOUT:   │ ├─VariablePattern 'var'
+// CHECK:STDOUT:   │ ├─VariableInitializer '='
+// CHECK:STDOUT:   │ │   ╭─IdentifierNameExpr 'x'
+// CHECK:STDOUT:   │ │ ╭─PrefixOperatorExpand 'expand'
+// CHECK:STDOUT:   │ ├─PrefixOperatorStar '*'
+// CHECK:STDOUT:   ├─VariableDecl ';'
+// CHECK:STDOUT:   │ ╭─VariableIntroducer 'var'
+// CHECK:STDOUT:   │ │   ╭─IdentifierNameNotBeforeSignature 'h'
+// CHECK:STDOUT:   │ │   ├─BindingPatternTypeStart ':'
+// CHECK:STDOUT:   │ │   ├─AutoTypeLiteral 'auto'
+// CHECK:STDOUT:   │ │ ╭─VarBindingPattern ':'
+// CHECK:STDOUT:   │ ├─VariablePattern 'var'
+// CHECK:STDOUT:   │ ├─VariableInitializer '='
+// CHECK:STDOUT:   │ │   ╭─IdentifierNameExpr 'x'
+// CHECK:STDOUT:   │ │ ╭─PrefixOperatorExpand 'expand'
+// CHECK:STDOUT:   │ ├─PrefixOperatorExpand 'expand'
+// CHECK:STDOUT:   ├─VariableDecl ';'
+// CHECK:STDOUT:   │ ╭─VariableIntroducer 'var'
+// CHECK:STDOUT:   │ │   ╭─IdentifierNameNotBeforeSignature 'i'
+// CHECK:STDOUT:   │ │   ├─BindingPatternTypeStart ':'
+// CHECK:STDOUT:   │ │   ├─AutoTypeLiteral 'auto'
+// CHECK:STDOUT:   │ │ ╭─VarBindingPattern ':'
+// CHECK:STDOUT:   │ ├─VariablePattern 'var'
+// CHECK:STDOUT:   │ ├─VariableInitializer '='
+// CHECK:STDOUT:   │ │   ╭─IdentifierNameExpr 'x'
+// CHECK:STDOUT:   │ │ ╭─PrefixOperatorExpand 'expand'
+// CHECK:STDOUT:   │ ├─PrefixOperatorMinus '-'
+// CHECK:STDOUT:   ├─VariableDecl ';'
+// CHECK:STDOUT:   ├─FileEnd ''
+// CHECK:STDOUT:   (root)
+// CHECK:STDOUT: - filename: fail_under_not.carbon
+// CHECK:STDOUT:   ╭─FileStart ''
+// CHECK:STDOUT:   │ ╭─VariableIntroducer 'var'
+// CHECK:STDOUT:   │ │   ╭─IdentifierNameNotBeforeSignature 'a'
+// CHECK:STDOUT:   │ │   ├─BindingPatternTypeStart ':'
+// CHECK:STDOUT:   │ │   ├─AutoTypeLiteral 'auto'
+// CHECK:STDOUT:   │ │ ╭─VarBindingPattern ':'
+// CHECK:STDOUT:   │ ├─VariablePattern 'var'
+// CHECK:STDOUT:   │ ├─VariableInitializer '='
+// CHECK:STDOUT:   │ │   ╭─IdentifierNameExpr 'x'
+// CHECK:STDOUT:   │ │ ╭─PrefixOperatorExpand 'expand'
+// CHECK:STDOUT:   │ ├─PrefixOperatorNot 'not'
+// CHECK:STDOUT:   ├─VariableDecl ';'
+// CHECK:STDOUT:   ├─FileEnd ''
+// CHECK:STDOUT:   (root)
+// CHECK:STDOUT: - filename: fail_as_postfix.carbon
+// CHECK:STDOUT:   ╭─FileStart ''
+// CHECK:STDOUT:   │ ╭─VariableIntroducer 'var'
+// CHECK:STDOUT:   │ │   ╭─IdentifierNameNotBeforeSignature 'a'
+// CHECK:STDOUT:   │ │   ├─BindingPatternTypeStart ':'
+// CHECK:STDOUT:   │ │   ├─AutoTypeLiteral 'auto'
+// CHECK:STDOUT:   │ │ ╭─VarBindingPattern ':'
+// CHECK:STDOUT:   │ ├─VariablePattern 'var'
+// CHECK:STDOUT:   │ ├─VariableInitializer '='
+// CHECK:STDOUT:   │ ├─IdentifierNameExpr 'x'
+// CHECK:STDOUT:   ├─VariableDecl ';' has_error
+// CHECK:STDOUT:   ├─FileEnd ''
+// CHECK:STDOUT:   (root)

toolchain/parse/testdata/variadics/pack_expansion_statement.carbon

@@ -0,0 +1,197 @@
+// 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
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/parse/testdata/variadics/pack_expansion_statement.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/parse/testdata/variadics/pack_expansion_statement.carbon
+
+// --- expr_statement.carbon
+
+fn F() {
+  ... G(expand x);
+}
+
+// --- var_statement.carbon
+
+fn F() {
+  ... var y: auto = expand x;
+}
+
+// --- compound_statements.carbon
+
+fn F() {
+  ... if (expand c) {
+    G(expand x);
+  } else {
+    H(expand y);
+  }
+  ... while (expand c) {}
+  ... return;
+}
+
+// --- nested.carbon
+
+fn F() {
+  ... ... G(expand x);
+}
+
+// --- fail_missing_statement.carbon
+
+fn F() {
+  ...
+// CHECK:STDERR: fail_missing_statement.carbon:[[@LINE+4]]:1: error: expected expression [ExpectedExpr]
+// CHECK:STDERR: }
+// CHECK:STDERR: ^
+// CHECK:STDERR:
+}
+
+// --- fail_missing_semi.carbon
+
+fn F() {
+  ... G()
+// CHECK:STDERR: fail_missing_semi.carbon:[[@LINE+4]]:1: error: expected `;` after expression statement [ExpectedExprSemi]
+// CHECK:STDERR: }
+// CHECK:STDERR: ^
+// CHECK:STDERR:
+}
+
+// CHECK:STDOUT: - filename: expr_statement.carbon
+// CHECK:STDOUT:   ╭─FileStart ''
+// CHECK:STDOUT:   │   ╭─FunctionIntroducer 'fn'
+// CHECK:STDOUT:   │   ├─IdentifierNameMaybeBeforeSignature 'F'
+// CHECK:STDOUT:   │   │ ╭─ExplicitParamListStart '('
+// CHECK:STDOUT:   │   ├─ExplicitParamList ')'
+// CHECK:STDOUT:   │ ╭─FunctionDefinitionStart '{'
+// CHECK:STDOUT:   │ │ ╭─PackExpansionStart '...'
+// CHECK:STDOUT:   │ │ │     ╭─IdentifierNameExpr 'G'
+// CHECK:STDOUT:   │ │ │   ╭─CallExprStart '('
+// CHECK:STDOUT:   │ │ │   │ ╭─IdentifierNameExpr 'x'
+// CHECK:STDOUT:   │ │ │   ├─PrefixOperatorExpand 'expand'
+// CHECK:STDOUT:   │ │ │ ╭─CallExpr ')'
+// CHECK:STDOUT:   │ │ ├─ExprStatement ';'
+// CHECK:STDOUT:   │ ├─PackExpansionStatement '...'
+// CHECK:STDOUT:   ├─FunctionDefinition '}'
+// CHECK:STDOUT:   ├─FileEnd ''
+// CHECK:STDOUT:   (root)
+// CHECK:STDOUT: - filename: var_statement.carbon
+// CHECK:STDOUT:   ╭─FileStart ''
+// CHECK:STDOUT:   │   ╭─FunctionIntroducer 'fn'
+// CHECK:STDOUT:   │   ├─IdentifierNameMaybeBeforeSignature 'F'
+// CHECK:STDOUT:   │   │ ╭─ExplicitParamListStart '('
+// CHECK:STDOUT:   │   ├─ExplicitParamList ')'
+// CHECK:STDOUT:   │ ╭─FunctionDefinitionStart '{'
+// CHECK:STDOUT:   │ │ ╭─PackExpansionStart '...'
+// CHECK:STDOUT:   │ │ │ ╭─VariableIntroducer 'var'
+// CHECK:STDOUT:   │ │ │ │   ╭─IdentifierNameNotBeforeSignature 'y'
+// CHECK:STDOUT:   │ │ │ │   ├─BindingPatternTypeStart ':'
+// CHECK:STDOUT:   │ │ │ │   ├─AutoTypeLiteral 'auto'
+// CHECK:STDOUT:   │ │ │ │ ╭─VarBindingPattern ':'
+// CHECK:STDOUT:   │ │ │ ├─VariablePattern 'var'
+// CHECK:STDOUT:   │ │ │ ├─VariableInitializer '='
+// CHECK:STDOUT:   │ │ │ │ ╭─IdentifierNameExpr 'x'
+// CHECK:STDOUT:   │ │ │ ├─PrefixOperatorExpand 'expand'
+// CHECK:STDOUT:   │ │ ├─VariableDecl ';'
+// CHECK:STDOUT:   │ ├─PackExpansionStatement '...'
+// CHECK:STDOUT:   ├─FunctionDefinition '}'
+// CHECK:STDOUT:   ├─FileEnd ''
+// CHECK:STDOUT:   (root)
+// CHECK:STDOUT: - filename: compound_statements.carbon
+// CHECK:STDOUT:   ╭─FileStart ''
+// CHECK:STDOUT:   │   ╭─FunctionIntroducer 'fn'
+// CHECK:STDOUT:   │   ├─IdentifierNameMaybeBeforeSignature 'F'
+// CHECK:STDOUT:   │   │ ╭─ExplicitParamListStart '('
+// CHECK:STDOUT:   │   ├─ExplicitParamList ')'
+// CHECK:STDOUT:   │ ╭─FunctionDefinitionStart '{'
+// CHECK:STDOUT:   │ │ ╭─PackExpansionStart '...'
+// CHECK:STDOUT:   │ │ │   ╭─IfConditionStart '('
+// CHECK:STDOUT:   │ │ │   │ ╭─IdentifierNameExpr 'c'
+// CHECK:STDOUT:   │ │ │   ├─PrefixOperatorExpand 'expand'
+// CHECK:STDOUT:   │ │ │ ╭─IfCondition ')'
+// CHECK:STDOUT:   │ │ │ │ ╭─CodeBlockStart '{'
+// CHECK:STDOUT:   │ │ │ │ │     ╭─IdentifierNameExpr 'G'
+// CHECK:STDOUT:   │ │ │ │ │   ╭─CallExprStart '('
+// CHECK:STDOUT:   │ │ │ │ │   │ ╭─IdentifierNameExpr 'x'
+// CHECK:STDOUT:   │ │ │ │ │   ├─PrefixOperatorExpand 'expand'
+// CHECK:STDOUT:   │ │ │ │ │ ╭─CallExpr ')'
+// CHECK:STDOUT:   │ │ │ │ ├─ExprStatement ';'
+// CHECK:STDOUT:   │ │ │ ├─CodeBlock '}'
+// CHECK:STDOUT:   │ │ │ ├─IfStatementElse 'else'
+// CHECK:STDOUT:   │ │ │ │ ╭─CodeBlockStart '{'
+// CHECK:STDOUT:   │ │ │ │ │     ╭─IdentifierNameExpr 'H'
+// CHECK:STDOUT:   │ │ │ │ │   ╭─CallExprStart '('
+// CHECK:STDOUT:   │ │ │ │ │   │ ╭─IdentifierNameExpr 'y'
+// CHECK:STDOUT:   │ │ │ │ │   ├─PrefixOperatorExpand 'expand'
+// CHECK:STDOUT:   │ │ │ │ │ ╭─CallExpr ')'
+// CHECK:STDOUT:   │ │ │ │ ├─ExprStatement ';'
+// CHECK:STDOUT:   │ │ │ ├─CodeBlock '}'
+// CHECK:STDOUT:   │ │ ├─IfStatement 'if'
+// CHECK:STDOUT:   │ ├─PackExpansionStatement '...'
+// CHECK:STDOUT:   │ │ ╭─PackExpansionStart '...'
+// CHECK:STDOUT:   │ │ │   ╭─WhileConditionStart '('
+// CHECK:STDOUT:   │ │ │   │ ╭─IdentifierNameExpr 'c'
+// CHECK:STDOUT:   │ │ │   ├─PrefixOperatorExpand 'expand'
+// CHECK:STDOUT:   │ │ │ ╭─WhileCondition ')'
+// CHECK:STDOUT:   │ │ │ │ ╭─CodeBlockStart '{'
+// CHECK:STDOUT:   │ │ │ ├─CodeBlock '}'
+// CHECK:STDOUT:   │ │ ├─WhileStatement 'while'
+// CHECK:STDOUT:   │ ├─PackExpansionStatement '...'
+// CHECK:STDOUT:   │ │ ╭─PackExpansionStart '...'
+// CHECK:STDOUT:   │ │ │ ╭─ReturnStatementStart 'return'
+// CHECK:STDOUT:   │ │ ├─ReturnStatement ';'
+// CHECK:STDOUT:   │ ├─PackExpansionStatement '...'
+// CHECK:STDOUT:   ├─FunctionDefinition '}'
+// CHECK:STDOUT:   ├─FileEnd ''
+// CHECK:STDOUT:   (root)
+// CHECK:STDOUT: - filename: nested.carbon
+// CHECK:STDOUT:   ╭─FileStart ''
+// CHECK:STDOUT:   │   ╭─FunctionIntroducer 'fn'
+// CHECK:STDOUT:   │   ├─IdentifierNameMaybeBeforeSignature 'F'
+// CHECK:STDOUT:   │   │ ╭─ExplicitParamListStart '('
+// CHECK:STDOUT:   │   ├─ExplicitParamList ')'
+// CHECK:STDOUT:   │ ╭─FunctionDefinitionStart '{'
+// CHECK:STDOUT:   │ │ ╭─PackExpansionStart '...'
+// CHECK:STDOUT:   │ │ │ ╭─PackExpansionStart '...'
+// CHECK:STDOUT:   │ │ │ │     ╭─IdentifierNameExpr 'G'
+// CHECK:STDOUT:   │ │ │ │   ╭─CallExprStart '('
+// CHECK:STDOUT:   │ │ │ │   │ ╭─IdentifierNameExpr 'x'
+// CHECK:STDOUT:   │ │ │ │   ├─PrefixOperatorExpand 'expand'
+// CHECK:STDOUT:   │ │ │ │ ╭─CallExpr ')'
+// CHECK:STDOUT:   │ │ │ ├─ExprStatement ';'
+// CHECK:STDOUT:   │ │ ├─PackExpansionStatement '...'
+// CHECK:STDOUT:   │ ├─PackExpansionStatement '...'
+// CHECK:STDOUT:   ├─FunctionDefinition '}'
+// CHECK:STDOUT:   ├─FileEnd ''
+// CHECK:STDOUT:   (root)
+// CHECK:STDOUT: - filename: fail_missing_statement.carbon
+// CHECK:STDOUT:   ╭─FileStart ''
+// CHECK:STDOUT:   │   ╭─FunctionIntroducer 'fn'
+// CHECK:STDOUT:   │   ├─IdentifierNameMaybeBeforeSignature 'F'
+// CHECK:STDOUT:   │   │ ╭─ExplicitParamListStart '('
+// CHECK:STDOUT:   │   ├─ExplicitParamList ')'
+// CHECK:STDOUT:   │ ╭─FunctionDefinitionStart '{'
+// CHECK:STDOUT:   │ │ ╭─PackExpansionStart '...'
+// CHECK:STDOUT:   │ │ │ ╭─InvalidParse '}' has_error
+// CHECK:STDOUT:   │ │ ├─ExprStatement '...' has_error
+// CHECK:STDOUT:   │ ├─PackExpansionStatement '...'
+// CHECK:STDOUT:   ├─FunctionDefinition '}'
+// CHECK:STDOUT:   ├─FileEnd ''
+// CHECK:STDOUT:   (root)
+// CHECK:STDOUT: - filename: fail_missing_semi.carbon
+// CHECK:STDOUT:   ╭─FileStart ''
+// CHECK:STDOUT:   │   ╭─FunctionIntroducer 'fn'
+// CHECK:STDOUT:   │   ├─IdentifierNameMaybeBeforeSignature 'F'
+// CHECK:STDOUT:   │   │ ╭─ExplicitParamListStart '('
+// CHECK:STDOUT:   │   ├─ExplicitParamList ')'
+// CHECK:STDOUT:   │ ╭─FunctionDefinitionStart '{'
+// CHECK:STDOUT:   │ │ ╭─PackExpansionStart '...'
+// CHECK:STDOUT:   │ │ │     ╭─IdentifierNameExpr 'G'
+// CHECK:STDOUT:   │ │ │   ╭─CallExprStart '('
+// CHECK:STDOUT:   │ │ │ ╭─CallExpr ')'
+// CHECK:STDOUT:   │ │ ├─ExprStatement ')' has_error
+// CHECK:STDOUT:   │ ├─PackExpansionStatement '...'
+// CHECK:STDOUT:   ├─FunctionDefinition '}'
+// CHECK:STDOUT:   ├─FileEnd ''
+// CHECK:STDOUT:   (root)

toolchain/parse/typed_nodes.h

@@ -954,6 +954,25 @@ struct WhileStatement {
   CodeBlockId body;
 };
 
+// The start of a pack expansion: `...`. This marks the beginning of the
+// expanded region.
+using PackExpansionStart =
+    LeafNode<NodeKind::PackExpansionStart, Lex::PeriodPeriodPeriodTokenIndex>;
+
+// A statement pack expansion: `... statement`.
+struct PackExpansionStatement {
+  static constexpr auto Kind = NodeKind::PackExpansionStatement.Define(
+      {.category = NodeCategory::Statement,
+       .bracketed_by = PackExpansionStart::Kind,
+       .child_count = 2});
+
+  PackExpansionStartId start;
+  AnyStatementId body;
+  // This is a virtual token. The `...` token is owned by the
+  // PackExpansionStart node.
+  Lex::PeriodPeriodPeriodTokenIndex token;
+};
+
 using MatchConditionStart =
     LeafNode<NodeKind::MatchConditionStart, Lex::OpenParenTokenIndex>;