Re: [PATCH 03/10] fast-export: use value from correct enum

[Jeff King <peff@peff.net>]

On Sun, Nov 11, 2018 at 09:10:17PM +0100, Ævar Arnfjörð Bjarmason wrote:

> > This is a good argument for naming these SIGNED_TAG_ABORT, etc. But this
> > is obviously an improvement in the meantime.
> 
> In C enum values aren't the types of the enum, but I'd thought someone
> would have added a warning for this:
> 
>     #include <stdio.h>
> 
>     enum { A, B } foo = A;
>     enum { C, D } bar = C;
> 
>     int main(void)
>     {
>         switch (foo) {
>           case C:
>             puts("A");
>             break;
>           case B:
>             puts("B");
>             break;
>         }
>     }
> 
> But none of the 4 C compilers (gcc, clang, suncc & xlc) I have warn
> about it. Good to know.

There is -Wenum-compare, but it does not seem to catch this (and is
enabled by -Wall). It (gcc, at least) does catch:

	enum foo { A, B };
	enum bar { C, D };

	int f(enum foo x)
	{
		return x == C;
	}

but converting that equality check to:

	switch (x) {
	case C:
		return 1;
	default:
		return 0;
	}

is not (which is essentially the same as your snippet). So I think the
bug / feature request is to have -Wenum-compare apply to switch
statements.

Clang has -Wenum-compare-switch, but I cannot seem to get it to complain
about even the "==" version using -Wenum-compare. Not sure if it's
buggy, or if I'm holding it wrong. This patch seems to be what we want:

  https://reviews.llvm.org/D36407

-Peff