Spurious -Wreturn-type warning with "pathological" for

[llvmbot]

Bugzilla Link	39026
Version	6.0
OS	Linux
Reporter	LLVM Bugzilla Contributor
CC	@DougGregor

Extended Description

Consider the following code:

int foo()
{
    for (int y = 0; !y;)
        for (/*decl*/; !y; ++y)
            return 1;
}

This generates a -Wreturn-type warning, despite that the inner loop body will always execute. Moreover, with optimization enabled, the compiler does (as expected) successfully remove the loops entirely.

(This is a simplified version of a pre-C++17 with statement. The purpose of this code, which is usually a macro, is to look like the opening statement of a block, where /*decl*/ — omitted in this example — is in scope only until the end of the block. FWIW, the C++17 form, if (/*decl*/; true) does not exhibit the problem.)

[wheatman]

This issue still exists in post 16 trunk(f05b58a)
https://godbolt.org/z/s59r9ne1W

The simplified code is

int foo() {
    for (int y = 0; y < 1; y++)
        return 1;
}

Where even though the loop will always execute and thus the function just returns 1, which it does determine with -O1
The warning still is returned

<source>:4:1: warning: non-void function does not return a value in all control paths [-Wreturn-type]
    4 | }
      | ^
1 warning generated.
Compiler returned: 0

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[AaronBallman]

Resolving as a duplicate of #111509