[cfe-dev] PR9824: -Wunused-but-set

[Nico Weber]

On Wed, Oct 24, 2012 at 10:02 AM, David Blaikie <dblaikie at gmail.com> wrote:
> I'm just looking at PR9824 now & starting with some sanity checking of
> existing related warnings.
>
> One result of fixing PR11550 was that given this (taken from
> test/SemaCXX/warn-unused-variables.cpp):
>
>   struct S1 { S1(); };
>   S1 makeS1();
>
> we warn on this:
>
>   S1 x = makeS1();
>
> under the reasoning that the user could just drop the named variable
> and call the function alone: makeS1();
>
> but we don't warn on this:
>
>   S1 x;
>
> even though the equivalent suggestion would be to unname this variable
> & simply invoke the default ctor: S1();
>
> Is that reasonable/acceptable/correct?

This feels right to me. Calling a constructor feels more "normal" than
invoking a constructor directly. It's a subjective thing, so clang
shouldn't warn about either I think.

> (Should we perhaps have a fixit
> in the first case explaining that the user should just call the
> function instead? Perhaps that's too hard to write in the presence of
> multiple declarators)
>
> From the examples in the bug, GCC doesn't warn about unused-but-set
> for code like this:
>
>    int i = 0;  // no warning
>    i = 1;
>    &x; // warning about unused-value here
>
> But it seems it'd be useful to indicate that the initializationg of
> zero is completely unused when it's overwritten with 1 in all cases
> anyway. Does that seem reasonable?
>
> Ted: you mentioned there's already a dead-store checker in the SA? Was
> your intent to basically port that up to the compiler proper? Are
> there major performance concerns about it/refactoring that would be
> required? Or will it just have to be basically reimplemented from
> scratch for it to be compiler-appropriate?
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