Re: [PATCH] libstdc++: Fix for non-constexpr math_errhandling

[Jonathan Wakely <jwakely.gcc@gmail.com>]

On Wed, 19 Jan 2022, 12:45 Matthias Kretz, <m.kretz@gsi.de> wrote:

> On Wednesday, 19 January 2022 13:07:26 CET Jonathan Wakely wrote:
> > On Wed, 19 Jan 2022 at 08:10, Matthias Kretz wrote:
> > > Follow-up to my last patch. This one is a more thorough fix. Tested on
> > > x86_64-
> > > linux. OK for trunk?
> > >
> > > ---- 8< ----
> > >
> > > Use SFINAE magic to support: "It is unspecified whether
> math_errhandling
> > > is a macro or an identifier with external linkage." [C Standard]
> >
> > The patch is OK for trunk, but I don't understand what the C standard
> means
> > here.
> >
> > "the macro math_errhandling expands to [...]. It is unspecified whether
> > math_errhandling is a macro or an identifier with external linkage."
> >
> > So is it a macro or not?
>
> I agree the quote I used is unclear. The complete paragraph:
>
> The macros
>
> MATH_ERRNO
> MATH_ERREXCEPT
>
> expand to the integer constants 1 and 2, respectively; the macro
>
> math_errhandling
>
> expands to an expression that has type int and the value MATH_ERRNO,
> MATH_ERREXCEPT, or the bitwise OR of both. The value of math_errhandling
> is
> constant for the duration of the program. It is unspecified whether
> math_errhandling is a macro or an identifier with external linkage. If a
> macro
> definition is suppressed or a program defines an identifier with the name
> math_errhandling, the behavior is undefined. If the expression
> math_errhandling & MATH_ERREXCEPT can be nonzero, the implementation shall
> define the macros FE_DIVBYZERO, FE_INVALID, and FE_OVERFLOW in
> <fenv.h>.
>


But that still says "the macro math_errhandling" and then says it might not
be a macro.

I'll ask some WG14 people for clarity, but it doesn't affect your patch.