[Bug c++/79433] __has_include reports wrong result for std headers that #error on __cplusplus

[egall at gwmail dot gwu.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79433

Eric Gallager <egall at gwmail dot gwu.edu> changed:

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                 CC|                            |egall at gwmail dot gwu.edu

--- Comment #6 from Eric Gallager <egall at gwmail dot gwu.edu> ---
(In reply to Marc Mutz from comment #5)
> Andrew, you're taking the easy way out. It might be that it works as
> implemented, but that behaviour is useless.
> 
> Please explain how one should detect, in a portable way, whether string_view
> and experimental::string_view is available, if not by headers check.

Do an autoconf test for it, AC_CHECK_HEADERS tests whether including the header
actually compiles.