[Parse] Error if closure has an unnamed parameter

[ahoppen]

We accepted unnamed closure parameters if the type was an array literal, dictionary literal, tuple or function (because the [ or ( starting the type was sufficient to disambiguate the type from the parameter’s name). This was never an accepted syntax and we should disallow it.

[ahoppen]

@swift-ci Please smoke test

[ahoppen]

@swift-ci Please test source compatibility

[AnthonyLatsis]

dd1b157 suggests the warning was introduced for source compatibility. Would it be better to keep it as a pre Swift 6 warning, and tweak the message to mention the upcoming source break, especially since we ended up accepting way more than we intended?

[hborla]

Would it be better to keep it as a pre Swift 6 warning, and tweak the message to mention the upcoming source break?

I suggested that Alex go ahead and make this an error as long as the source compatibility test turns up green. This diagnostic has been staged in as a warning for a long time, and the last known adoption of this invalid syntax, which prompted dd1b157, has since been changed. I think that if we see any fallout from this change, we should use .warnUntilSwiftVersion(6), but if not, it's okay to make this an error now. It's beneficial for SwiftSyntax that the syntax tree of closure parameter lists is not conditional based on language version.