[Bug c/66425] (void) cast doesn't suppress __attribute__((warn_unused_result))

[joseph at codesourcery dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66425

--- Comment #16 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
I'd say that for any function for which use of this attribute is 
appropriate, suppression of the warning should involve a detailed comment 
explaining why the particular use of the function is so exceptional that, 
very unusually, it is not possible to (for example) do anything useful 
with the information that the function call failed or it is otherwise 
exceptionally safe to ignore the result - and that any suppression of the 
warning should involve extra-careful code review when added.

See, for example, how glibc handles use of diagnostic suppression pragmas, 
via macros DIAG_PUSH_NEEDS_COMMENT, DIAG_IGNORE_NEEDS_COMMENT and 
DIAG_POP_NEEDS_COMMENT, to make it extra-obvious if a patch is adding such 
uses and to make it obvious at the use site that such comments are needed.

Now, the compiler cannot check whether an explanatory comment justifying 
diagnostic suppression is present, and cannot know how a particular 
project wishes to handle justifying exceptions to its normal coding style 
rules.  But it can make it as visible as possible that something unusual 
and dubious is being done by ignoring the return value, to reduce the 
chance of it slipping by reviewers.

I'd say that a warning suppressed by a (void) cast should be a separate 
attribute, e.g. warn_unused_result_weak.