Keywords like `case` cannot be used, unescaped, as expression macro names

[stephencelis]

Description

I was hoping keywords like case could be used as expression macro names in an unescaped manner, like static functions and methods, but sadly that doesn't seem to be the case.

Steps to reproduce

extension Int {
  static let `case` = 1
}

let _: Int = .case   // ✅

@freestanding(expression)
public macro `case`(Int) -> Int = …

let _ = #case(42)    // 🛑
let _ = #`case`(42)  // ✅

Expected behavior

I expect #case(42) to compile OK.

Environment

• Swift compiler version info

  swift-driver version: 1.82.2 Apple Swift version 5.9 (swiftlang-5.9.0.114.6 clang-1500.0.27.1)
  Target: arm64-apple-macosx13.0
  

• Xcode version info

  Xcode 15.0
  Build version 15A5160n
  

[AnthonyLatsis]

cc @ahoppen

In case this also applies to the new parser.

[ahoppen]

The new parser has the same issue

swift-parser-cli print-diags -s '#case'
=== stdin:1 ===
1 │ #case
  │  ╰─ error: keyword 'case' cannot be used as an identifier here

[ahoppen]

Tracked in Apple’s issue tracker as rdar://110472060.