[6.1 regression]: #if directives may suppress some warning emissions

[jamieQ]

Description

No response

Reproduction

func f() {
    // expected unused variable warning
    var x = 0   // ⚠️ initialization of variable 'x' was never used; consider replacing with assignment to '_' or removing it
}

func g() {
    // no unused variable warnings produced
    #if true
    var x = 0   // ❓no warning – unexpected
    #else
    var x = 0   // no warning – expected
    #endif
}

Expected behavior

warnings in the evaluated conditional compilation branches should still be produced

Environment

Swift version 6.3-dev (LLVM cff341857efbfc7, Swift 956dc0c)
Target: x86_64-unknown-linux-gnu

Additional information

i didn't go back in history too far, but the issue affecting the repro appears to have started in the 6.1 release.
godbolt: https://swift.godbolt.org/z/9oebPGoo1

[slavapestov]

This is expected behavior, because inactive #if blocks are skipped more aggressively now. The old behavior was pretty bogus anyway.

[jamieQ]

This is expected behavior, because inactive #if blocks are skipped more aggressively now. The old behavior was pretty bogus anyway.

@slavapestov i don't follow... if the inactive blocks are skipped more aggressively, why would we lose the diagnostic? are the branches being entirely skipped? the old behavior correctly produced the diagnostic in the active conditional branch, and now it's just not there: https://swift.godbolt.org/z/Y11an31sK.