Re: [PATCH] Amend AC_CHECK_HEADER_STDBOOL for C++11 and modern dialects

[David Seifert]

On Mi, 2016-06-01 at 11:22 -0700, Paul Eggert wrote:
> On 06/01/2016 01:29 AM, David Seifert wrote:
> > 
> > stdbool.m4 is supposed to merely check that
> > stdbool.h fulfills C99 requirements. Why does it then necessarily 
> > check whether bool is a
> > defined _macro_?
> Because C99 (and C11) require bool to be a macro.
> 
> m4/stdbool.m4 now does that check only inside #if __cplusplus <
> 201103, 
> so I don't see why clang++ would complain now. Is it setting
> __cplusplus 
> to an older value? Do older versions of the C++ standard require bool
> to 
> be a macro?  (I don't use C++.)

Clang++ hasn't made the switch to a post -std=c++98 world yet. This
means that literally every ./configure with CC=clang++ will fail. The
crux of the matter is that clang/LLVM will only ever define
bool/true/false as macros if __GNUC__ is defined, which is imo
unrealistic. C++98 never required nor explicitly defined
bool/true/false to be macros, mainly because they are proper
types/literals in C++.

> > 
> > Wouldn't it be better to change the stdbool.m4 macro to instead
> > try 
> > and use "bool" in a statement
> > or as a type
> It depends on how strict the compatibility checking should be. Kind
> of a 
> judgment call, I suppose. That being said, I don't see the harm in
> the 
> current approach for C. Maybe it needs to be toned down for C++, but
> I'd 
> like to know why before fiddling with this (it's been this way since
> at 
> least 2002...).
> 

We could relax the whole thing a bit, and maybe just make a plain
__cplusplus check, without a version number. This keeps the C side as
it always has been, but allows C++'s proper literals to kick in.

In addition, _Bool is also a major obstacle, as it does not exist in
C++ (and does so only for GCC due to some GNUisms in libstdc++ with
-std=c++98). Would you consider moving away from _Bool and replacing
all uses of it with bool? After all, <stdbool.h> by itself only
mandates the existence of bool/true/false.