[Bug tree-optimization/109717] -Warray-bound error with gnu++20 and fmt library

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109717

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |dmalcolm at gcc dot gnu.org

--- Comment #8 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #7)
> And requiring every library to annotate every assumption causes regressions
> like PR 109703, where the cure is worse than the disease.
> 
> A wrong-code bug is worse than a false positive warning, but the warnings
> are forcing people to modify correct code.

I agree, but then I do not see how to reliably diagnose real problems.  There
is no way GCC can prove a line of code will be executed (the whole function
could be dead).  And given C++ abstraction and high branch density the
amount of IL not under conditional execution in a function is about zero.

Annotating the library also gives way to better optimization.

It might be an interesting experiment to use coverage data to prune
diagnostics on locations that are not covered.  All our late diagnostics
could be put into a separate -Wall.  That of course requires we can
somehow reliably get coverage of a program and also distinguish the
various copies of the same location passes like jump threading create ...

Might be also interesting to steer static analysis to those "interesting"
paths.

One requirement would probably be that we put all late diagnostics on
a common point in the pass pipeline, otherwise adding discriminators for
all "variants" of the IL we run into makes the coverage data explode
(much easier for the analyzer here).