Swift warning `for let/var in`

I compiled swift for Debian (LLVM f63b283c71, Clang 41ac4c4262, Swift 7cb2c9bbba) and get a strange error/warning-pair with for let/var in:

%> cat l.swift
import Foundation

func f(_ data: Data) -> Void {
   for let x in data {
      print("\(x)")
   }
}
%> swift l.swift
l.swift:4:8: error: 'let' pattern cannot appear nested in an already immutable context
   for let x in data {
       ^

If it is an immutable context, then why not allow let? But ok, let’s use var:

%> cat v.swift
import Foundation

func f(_ data: Data) -> Void {
   for var x in data {
      print("\(x)")
   }
}
%> swift v.swift
v.swift:4:12: warning: variable 'x' was never mutated; consider changing to 'let' constant
   for var x in data {
       ~~~~^

There is no warning for var in REPL:

%> swift
Welcome to Swift version 5.0-dev (LLVM f63b283c71, Clang 41ac4c4262, Swift 7cb2c9bbba).
Type :help for assistance.
  1> import Foundation
  2.
  3. func f(_ data: Data) -> Void {
  4.    for var x in data {
  5.       print("\(x)")
  6.    }
  7. }
  8>
1 Like
for x in data {
  print("\(x)")
}

So the diagnostic message is misleading, perhaps that can be improved? Xcode on macOS presents the same warning, but the “Fix-it” (correctly) removes the var .

4 Likes

Please file a bug!

Which Swift version are you using exactly? I cannot yet reproduce the warning on Linux.

Will do.

5.0-dev
(LLVM f63b283c71, Clang 41ac4c4262, Swift 7cb2c9bbba)

That was my fault. I had tried it in the REPL and not – as you clearly wrote – by calling swift v.swift. The REPL does not show a warning, but calling swift with a source file argument does.

Done: SR-9732

2 Likes
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