Re: [PATCH] c++, v2: Implement C++23 P2647R1 - Permitting static constexpr variables in constexpr functions

[Jonathan Wakely <jwakely@redhat.com>]

On Tue, 15 Nov 2022 at 23:36, Jason Merrill <jason@redhat.com> wrote:
>
> On 11/13/22 01:45, Jakub Jelinek wrote:
> > On Fri, Nov 11, 2022 at 06:07:04PM +0100, Jakub Jelinek wrote:
> >> The following patch on top of Marek's P2448 PR106649 patch
> >> (mainly because that patch implements the previous __cpp_constexpr
> >> feature test macro bump so this can't go in earlier; OT,
> >> P2280R4 doesn't have any feature test macro?) implements this
> >> simple paper.
> >>
> >> Ok for trunk if it passes bootstrap/regtest and is voted into C++23?
> >
> > Here is an updated patch that passed bootstrap/regtest, the only
> > change is another testcase tweak.
> >
> > 2022-11-13  Jakub Jelinek  <jakub@redhat.com>
> >
> > gcc/c-family/
> >       * c-cppbuiltin.cc (c_cpp_builtins): Bump __cpp_constexpr
> >       value from 202207L to 202211L.
> > gcc/cp/
> >       * constexpr.cc (cxx_eval_constant_expression): Implement C++23
> >       P2647R1 - Permitting static constexpr variables in constexpr functions.
> >       Allow decl_maybe_constant_var_p static or thread_local vars for
> >       C++23.
>
> This was accepted as a DR, so it shouldn't be limited to C++23 mode.
> Certainly it should be allowed in C++20 mode; I don't have a strong
> opinion about C++14/17.  Jonathan, do you?

I don't. The lack of this feature caused some awkwardness implementing
a C++23 library feature in <charconv>, but that means my desire for it
was only in C++23 mode. And that has been mitigated by changes Patrick
made to work around it anyway.

I think it does make sense for C++20, where constexpr is pretty close
to "normal" code, what with dynamic allocations, std::string etc. but
I don't see a great need for it before C++20.

>
> >       (potential_constant_expression_1): Likewise.
> > gcc/testsuite/
> >       * g++.dg/cpp23/constexpr-nonlit17.C: New test.
> >       * g++.dg/cpp23/feat-cxx2b.C: Adjust expected __cpp_constexpr
> >       value.
> >       * g++.dg/ext/stmtexpr19.C: Don't expect an error for C++23 or later.
> >
> > --- gcc/c-family/c-cppbuiltin.cc.jj   2022-11-11 17:14:52.021613271 +0100
> > +++ gcc/c-family/c-cppbuiltin.cc      2022-11-11 17:17:45.065265246 +0100
> > @@ -1074,7 +1074,7 @@ c_cpp_builtins (cpp_reader *pfile)
> >         /* Set feature test macros for C++23.  */
> >         cpp_define (pfile, "__cpp_size_t_suffix=202011L");
> >         cpp_define (pfile, "__cpp_if_consteval=202106L");
> > -       cpp_define (pfile, "__cpp_constexpr=202207L");
> > +       cpp_define (pfile, "__cpp_constexpr=202211L");
> >         cpp_define (pfile, "__cpp_multidimensional_subscript=202110L");
> >         cpp_define (pfile, "__cpp_named_character_escapes=202207L");
> >         cpp_define (pfile, "__cpp_static_call_operator=202207L");
> > --- gcc/cp/constexpr.cc.jj    2022-11-11 17:14:52.024613231 +0100
> > +++ gcc/cp/constexpr.cc       2022-11-11 17:16:54.384952917 +0100
> > @@ -7085,7 +7085,8 @@ cxx_eval_constant_expression (const cons
> >           && (TREE_STATIC (r)
> >               || (CP_DECL_THREAD_LOCAL_P (r) && !DECL_REALLY_EXTERN (r)))
> >           /* Allow __FUNCTION__ etc.  */
> > -         && !DECL_ARTIFICIAL (r))
> > +         && !DECL_ARTIFICIAL (r)
> > +         && (cxx_dialect < cxx23 || !decl_maybe_constant_var_p (r)))
> >         {
> >           if (!ctx->quiet)
> >             {
> > @@ -9577,7 +9578,10 @@ potential_constant_expression_1 (tree t,
> >         tmp = DECL_EXPR_DECL (t);
> >         if (VAR_P (tmp) && !DECL_ARTIFICIAL (tmp))
> >       {
> > -       if (CP_DECL_THREAD_LOCAL_P (tmp) && !DECL_REALLY_EXTERN (tmp))
> > +       if (CP_DECL_THREAD_LOCAL_P (tmp)
> > +           && !DECL_REALLY_EXTERN (tmp)
> > +           && (cxx_dialect < cxx23
> > +               || !decl_maybe_constant_var_p (tmp)))
> >           {
> >             if (flags & tf_error)
> >               constexpr_error (DECL_SOURCE_LOCATION (tmp), fundef_p,
> > @@ -9585,7 +9589,9 @@ potential_constant_expression_1 (tree t,
> >                                "%<constexpr%> context", tmp);
> >             return false;
> >           }
> > -       else if (TREE_STATIC (tmp))
> > +       else if (TREE_STATIC (tmp)
> > +                && (cxx_dialect < cxx23
> > +                    || !decl_maybe_constant_var_p (tmp)))
> >           {
> >             if (flags & tf_error)
> >               constexpr_error (DECL_SOURCE_LOCATION (tmp), fundef_p,
> > --- gcc/testsuite/g++.dg/cpp23/constexpr-nonlit17.C.jj        2022-11-11 17:59:59.972852793 +0100
> > +++ gcc/testsuite/g++.dg/cpp23/constexpr-nonlit17.C   2022-11-11 17:59:38.725141231 +0100
> > @@ -0,0 +1,12 @@
> > +// P2647R1 - Permitting static constexpr variables in constexpr functions
> > +// { dg-do compile { target c++23 } }
> > +
> > +constexpr char
> > +test ()
> > +{
> > +  static const int x = 5;
> > +  static constexpr char c[] = "Hello World";
> > +  return *(c + x);
> > +}
> > +
> > +static_assert (test () == ' ');
> > --- gcc/testsuite/g++.dg/cpp23/feat-cxx2b.C.jj        2022-11-11 17:14:52.194610922 +0100
> > +++ gcc/testsuite/g++.dg/cpp23/feat-cxx2b.C   2022-11-11 17:48:56.038865825 +0100
> > @@ -134,8 +134,8 @@
> >
> >   #ifndef __cpp_constexpr
> >   #  error "__cpp_constexpr"
> > -#elif __cpp_constexpr != 202207
> > -#  error "__cpp_constexpr != 202207"
> > +#elif __cpp_constexpr != 202211
> > +#  error "__cpp_constexpr != 202211"
> >   #endif
> >
> >   #ifndef __cpp_decltype_auto
> > --- gcc/testsuite/g++.dg/ext/stmtexpr19.C.jj  2020-01-14 20:02:46.839608995 +0100
> > +++ gcc/testsuite/g++.dg/ext/stmtexpr19.C     2022-11-12 09:17:40.706245495 +0100
> > @@ -8,7 +8,7 @@ const test* setup()
> >   {
> >     static constexpr test atest =
> >       {
> > -      ({ static const int inner = 123; &inner; }) // { dg-error "static" }
> > +      ({ static const int inner = 123; &inner; }) // { dg-error "static" "" { target c++20_down } }
> >       };
> >
> >     return &atest;
> >
> >
> >       Jakub
> >
>