clang -Wconstant-conversion conditional operator false positive

[osandov]

The following C source code:

int x = 0 ? 9223372036854775807L : 2147483648;

Produces the following warning when compiled with Clang:

$ clang --version
clang version 16.0.6 (Fedora 16.0.6-2.fc38)
Target: x86_64-redhat-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
$ clang -Wconstant-conversion -fsyntax-only test.c
test.c:1:13: warning: implicit conversion from 'long' to 'int' changes value from 9223372036854775807 to -1 [-Wconstant-conversion]
int x = 0 ? 9223372036854775807L : 2147483647;
    ~       ^~~~~~~~~~~~~~~~~~~~
1 warning generated.

0 ? 9223372036854775807L : 2147483647 evaluates to 2147483647L, so this warning is incorrect. GCC's similar -Woverflow doesn't trigger for this test case.

[shafik]

This does feel a bit noisy, wdyt @AaronBallman

[nathanchance]

I suspect this is #38137, based on https://godbolt.org/z/c9E5bPGhP. The Linux kernel has hit this in a couple of places (ClangBuiltLinux/linux#92).

[AaronBallman]

@nathanchance is correct, this is a duplicate of #38137 (and boils down to trying to gin up a CFG at global scope), but also worth noting that -Woverflow in GCC also misses the overflow in the third operand: https://godbolt.org/z/fc98PPY76