[clang] [clang] Emit bad shift warnings (PR #70307)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> This started to cause
> 
> ```
> $ cat test.c
> #include <string.h>
> void f(void) {
>     char a[strlen("x")];
>     (void) a;
> }
> ```
> 
> ```
> $ clang -Wall -fsyntax-only test.c
> test.c:3:12: warning: variable length array folded to constant array as an extension [-Wgnu-folding-constant]
>     3 |     char a[strlen("x")];
>       |            ^~~~~~~~~~~
> 1 warning generated.
> ```
> 
> to emit a warning, and I'm not sure that's intentional?

That is intentional; `strlen` is not a valid constant expression in C, and so that is technically a VLA, except we have an extension to fold it back into a constant.

https://github.com/llvm/llvm-project/pull/70307