[Bug c++/95148] -Wtype-limits always-false warning triggered despite comparison being avoided

["eyalroz1 at gmx dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95148

--- Comment #4 from Eyal Rozenberg <eyalroz1 at gmx dot com> ---
(In reply to Jonathan Wakely from comment #3)

> > I should not be getting this warning, because when x is unsigned, the
> > comparison is never performed, due to the short-circuit semantics of `and`.
> 
> Those semantics only apply at runtime.

But the semantics are known at compile time, and so are the argument values.

> > No less
> > importantly, the author of such a line in a program clearly specified
> > his/her intent here with this check. 
> 
> But `x && y` doesn't prevent y being instantiated, it only prevents it being
> evaluated at runtime.

You mean, it only prevents a comparison of an unsigned and signed values to
actually ever being carried out? Yeah :-)


> >   a.cpp:5:52: warning: comparison of unsigned expression < 0 is always false
> > [-Wtype-limits]
> 
> Both branches of the condition must be instantiated by the compiler, and so
> this warning (which happens in the front-end, not after optimizing the AST)
> gets triggered.

Well, that's a bug. If you're saying that the front-end produces the warning,
then delay it to a later stage, in which you can figure out whether the warning
is called for. 

clang doesn't produce this warning.

> I think to avoid this warning the front end would have to disable this
> particular warning for branches that depend on a compile-time constant. I
> don't know if that's currently possible in GCC.

Ok, I didn't say that the solution would be easy...