Lookahead/parse mismatch: canParseClosureSignature accepts top-level '->' but signature parse fails (missing 'in')

[kwikysw]

Issue Kind

Bad Diagnostic Produced

Source Code

struct Foo {
func a(s: S [{{g) -> Int {}
}}}

Description

Following #3162, the following code in function testRecovery141 of RecoveryTests gives the following diagnostics: "expected 'in' in closure signature" at the arrow location, and "unexpected code ') -> Int {}' in closure". However, lookahead shouldn't accept a closure signature here, or the parse should consume consistently, instead of having a mismatch between lookahead and the actual parse.