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Recover an incomplete lambda as a complete expression. (#7435)

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A lambda introducer (`fn` in expression context) with no parameters and
no body -- for example `(fn)`, where `)` immediately follows `fn` --
left `HandleLambdaAfterParams` calling `ReturnErrorOnState()` without
ever emitting a `Lambda` node. The orphaned `LambdaIntroducer` leaf was
then left where an expression was required, so `Parse` produced a tree
that failed its own verification and aborted via `CARBON_FATAL`.

Recover the way `HandleLambdaBody` already does for a missing body after
a return type: emit a placeholder `InvalidParse` body and finish a
complete `Lambda` node, so the lambda stays a valid expression and the
surrounding construct (here a `ParenExpr`) extracts cleanly.

Found by fuzzing.

Assisted-by: Claude Code
This commit is contained in:
Chandler Carruth
2026-06-29 21:12:58 +00:00
committed by GitHub
parent baa91882dc
commit 366cb1010b
3 changed files with 31 additions and 4 deletions
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toolchain/parse/testdata/lambda/lambda.carbon
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@@ -155,9 +155,11 @@ fn G() {
// CHECK:STDOUT: {kind: 'VarBindingPattern', text: ':', subtree_size: 3},
// CHECK:STDOUT: {kind: 'VariablePattern', text: 'var', subtree_size: 4},
// CHECK:STDOUT: {kind: 'VariableInitializer', text: '='},
// CHECK:STDOUT: {kind: 'LambdaIntroducer', text: 'fn'},
// CHECK:STDOUT: {kind: 'VariableDecl', text: ';', has_error: yes, subtree_size: 8},
// CHECK:STDOUT: {kind: 'FunctionDefinition', text: '}', subtree_size: 14},
// CHECK:STDOUT: {kind: 'LambdaIntroducer', text: 'fn'},
// CHECK:STDOUT: {kind: 'InvalidParse', text: ';', has_error: yes},
// CHECK:STDOUT: {kind: 'Lambda', text: 'fn', has_error: yes, subtree_size: 3},
// CHECK:STDOUT: {kind: 'VariableDecl', text: ';', subtree_size: 10},
// CHECK:STDOUT: {kind: 'FunctionDefinition', text: '}', subtree_size: 16},
// CHECK:STDOUT: {kind: 'FileEnd', text: ''},
// CHECK:STDOUT: ]
// CHECK:STDOUT: - filename: fail_expected_body_after_return_type.carbon