[brandan-prnd]

Summary

-swift-version only accepts Swift language modes (4, 4.2, 5, 6), but SWIFT_VERSION in an Xcode project is an arbitrary x.y build setting (6.0, 5.9, 4.0, …). Xcode normalizes it before calling swiftc; axe only trimmed a trailing .0, so any other value was passed through verbatim and the preview thunk failed to compile.

This PR maps any x.y to its major version (keeping 4.2, which is a real language mode) and drops the special case that preserved 4.0.

Problem

Projects that set SWIFT_VERSION = 5.9 in an xcconfig cannot capture any preview:

invalid value '5.9' in '-swift-version 5.9'
note: valid arguments to '-swift-version' are '4', '4.2', '5', '6'

The previous normalization was:

if strings.HasSuffix(sv, ".0") && sv != "4.0" {
    sv = strings.TrimSuffix(sv, ".0")
}

It handles 6.0 → 6 and 5.0 → 5, but 5.9 (or 5.10, 6.1, …) is left untouched. The 4.0 exception also turns out to be wrong: swiftc rejects 4.0 exactly like 5.9.

Fix

// SWIFT_VERSION may be "6.0" or "5.9" but -swift-version only accepts "4", "4.2", "5", "6".
if major, _, ok := strings.Cut(sv, "."); ok && sv != "4.2" {
    sv = major
}

• Any x.y becomes x.

• 4.2 is kept as-is because it is a distinct language mode.

• The literal 4.0 is no longer preserved. Verified with Swift 6.3.3:

  -swift-version 4    OK
  -swift-version 4.0  error: invalid value '4.0' in '-swift-version 4.0'
  -swift-version 4.2  OK
  

Verification

• TestCompileThunk_SwiftVersionTrimming now covers 5.9 → 5 and 4.0 → 4 alongside the existing cases.
• Verified end to end on a project with SWIFT_VERSION=5.9: the thunk is compiled with -swift-version 5 and the capture succeeds.

[k-kohey]

Thanks for your contribution!