Parsing of closure parameter list without parameter name fails

[ahoppen]

Issue Kind

Parse of Valid Source Failed

Source Code

func test(_: ([Int]) -> Void) {}
test { ([Int]) in }

Description

The above code is valid in the old parser but not in SwiftParser.

[ahoppen]

Tracked in Apple’s issue tracker as rdar://118012839

[AnthonyLatsis]

Also e.g. ([X:Y]), ((X) -> Y), and (any X).

More generally, the old parser accepts (warns about rather than rejects) this kind of closure parameter production when all of the following are true:

• An argument name cannot be parsed (link)
• A type can be parsed per Parser::canParseType(), and said type is followed by ,, ), or = (link)
• Parsed type per Parser::parseType is neither OptionalTypeRepr nor ImplicitlyUnwrappedOptionalTypeRepr (link)

Ideally, I think a Swift parser should accept and recover from this so long as the type is unambiguous.

[ahoppen]

Omitting the parameter’s name has never been an officially accepted language feature and the Swift grammar also requires a parameter name (https://docs.swift.org/swift-book/documentation/the-swift-programming-language/expressions#Capture-Lists).

The warning in the C++ parser should be an error, which I’m changing in swiftlang/swift#70065. So, the new parser behaves correctly. Closing this issue.