Trailing comma not allowed in @available annotation despite SE-0439

[calda]

Description

Trailing commas are unexpectedly not allowed in @available annotations, despite annotations being mentioned as a supported use case in SE-0439.

@available(
    *,
    deprecated,
    renamed: "bar", // unexpectedly not allowed
)
func foo() { }

Reproduction

Compile the above code using Swift 6.1+: https://swiftfiddle.com/oiaoenmyxbcajdqv4m522kji3q

Expected behavior

It should compile but doesn't

Environment

Swift version 6.2-dev (LLVM a3e32cdaae72f3d, Swift fa86097)

Additional information

No response

[calda]

Issue also applies to any built-in attribute with parens, like @backDeployed, @attached, @freestanding, and @objc(...):

@backDeployed(
  before: iOS 17 // trailing comma not allowed
)
public func foo() {}

@freestanding(
  declaration,
  names: named(CodingKeys) // trailing comma not allowed
)
macro FreestandingMacro() = #externalMacro(module: "Macros", type: "")

@attached(
  extension,
  names: arbitrary // trailing comma not allowed
)
macro AttachedMacro() = #externalMacro(module: "Macros", type: "")

and I suspect also all of the underscored built-in attributes, e.g.:

@_originallyDefinedIn(
    module: "Foo",
    macOS 10.0  // trailing comma not allowed
)
struct Bar {}

[calda]

fyi @mateusrodriguesxyz

[calda]

Fixed in #81612

[calda]

This appears to be intentional: #81612 (comment)

[xwu]

Yeah, there's a bit of underspecification here with the proposal.

The bottom line is that we resolved to allow trailing commas where we have purely a comma-separated list within symmetric delimiters and we disallow trailing commas where commas aren't ever permitted within symmetric delimiters (for example: [Int,] in type position—there is an extant bug where we accept this in some contexts which the steering group felt was not reason for official adoption).

As you've noted, attributes can have custom parsing for their arguments, and during this proposal process no attempt was made to sort out which should or should not be considered purely comma-separated lists that support trailing commas—as this work was never done we have to consider it not part of the approved proposal.