[Bug c++/110765] New: [13 regression] constraints on parameters of derived type in CRTP base

[Bug c++/110765] New: [13 regression] constraints on parameters of derived type in CRTP base

[h2+bugs at fsfe dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110765

            Bug ID: 110765
           Summary: [13 regression] constraints on parameters of derived
                    type in CRTP base
           Product: gcc
           Version: 13.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: h2+bugs at fsfe dot org
  Target Milestone: ---

Created attachment 55595
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55595&action=edit
GCC12's intermediate code; build with -std=c++20

I have large parts of library code that fail to build with GCC13, although they
build with GCC10-12 and Clang-16.

I have attached GCC12's intermediate code that compiles cleanly with GCC12 but
fails to build with GCC13. The error is the same as when building the actual
code. See logs below.

Playing a bit around with the code seems to indicate that constrained friend
declarations in the CRTP base that references the derived class could be the
problem:

Triggering code:
```cpp
friend constexpr auto tag_invoke(custom::to_char, derived_type const a)
noexcept
    requires(requires { { a.to_char() }; })
{
    return a.to_char();
}
```

Workaround:
```cpp
friend constexpr auto tag_invoke(custom::to_char, std::same_as<derived_type>
auto const a) noexcept
    requires(requires { { a.to_char() }; })
{
    return a.to_char();
}
```

It seems that GCC13 tries to check the constraints on the derived class earlier
than previous GCCs and Clang, and apparently before the type is complete.


Passes:
% g++12 --version 
g++12 (FreeBSD Ports Collection) 12.2.0
%g++12 -std=c++20 a-test.ii
%

Fails:
% g++13 --version
g++13 (FreeBSD Ports Collection) 13.1.1 20230701
% g++13 -std=c++20 a-test.ii
In file included from /usr/local/lib/gcc12/include/c++/bits/stl_pair.h:60,
                 from /usr/local/lib/gcc12/include/c++/bits/stl_algobase.h:64,
                 from /usr/local/lib/gcc12/include/c++/vector:60,
                 from
/home/hannes/devel/biocpp-core/include/bio/alphabet/nucleotide/dna4.hpp:16,
                 from test.cpp:1:
/usr/local/lib/gcc12/include/c++/type_traits:1522:12: error: expected
identifier before '__is_nothrow_convertible'
 1522 |     struct __is_nothrow_convertible
      |            ^~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/lib/gcc12/include/c++/type_traits:1522:12: error: expected
unqualified-id before '__is_nothrow_convertible'
/usr/local/lib/gcc12/include/c++/type_traits:3083:66: error: template argument
2 is invalid
 3083 |             __is_nothrow_convertible<typename _Result::type, _Ret>>
      |                                                                  ^~
In file included from /usr/local/lib/gcc12/include/c++/memory:76,
                 from
/home/hannes/devel/biocpp-core/include/bio/meta/detail/type_inspection.hpp:21,
                 from
/home/hannes/devel/biocpp-core/include/bio/alphabet/concept.hpp:22,
                 from
/home/hannes/devel/biocpp-core/include/bio/alphabet/base.hpp:20,
                 from
/home/hannes/devel/biocpp-core/include/bio/alphabet/nucleotide/nucleotide_base.hpp:16,
                 from
/home/hannes/devel/biocpp-core/include/bio/alphabet/nucleotide/dna4.hpp:18:
/usr/local/lib/gcc12/include/c++/bits/unique_ptr.h:536:15: error: expected
identifier before '__remove_cv'
  536 |         using __remove_cv = typename remove_cv<_Up>::type;
      |               ^~~~~~~~~~~
/usr/local/lib/gcc12/include/c++/bits/unique_ptr.h:536:27: error: expected '('
before '=' token
  536 |         using __remove_cv = typename remove_cv<_Up>::type;
      |                           ^
      |                           (
/usr/local/lib/gcc12/include/c++/bits/unique_ptr.h:536:27: error: expected
type-specifier before '=' token
/usr/local/lib/gcc12/include/c++/bits/unique_ptr.h:536:27: error: expected
unqualified-id before '=' token
/usr/local/lib/gcc12/include/c++/bits/unique_ptr.h:542:69: error: wrong number
of template arguments (1, should be 2)
  542 |                     __not_<is_same<__remove_cv<_Tp>, __remove_cv<_Up>>>
>;
      |                                                                     ^~
/usr/local/lib/gcc12/include/c++/type_traits:635:12: note: provided for
'template<class, class> struct std::is_same'
  635 |     struct is_same;
      |            ^~~~~~~
/usr/local/lib/gcc12/include/c++/bits/unique_ptr.h:542:71: error: template
argument 1 is invalid
  542 |                     __not_<is_same<__remove_cv<_Tp>, __remove_cv<_Up>>>
>;
      |                                                                       ^
/usr/local/lib/gcc12/include/c++/bits/unique_ptr.h:542:73: error: template
argument 2 is invalid
  542 |                     __not_<is_same<__remove_cv<_Tp>, __remove_cv<_Up>>>
>;
      |                                                                        
^
/usr/local/lib/gcc12/include/c++/type_traits: In instantiation of 'struct
std::is_nothrow_destructible<bio::alphabet::dna4>':
/home/hannes/devel/biocpp-core/include/bio/alphabet/base.hpp:260:27:  
recursively required from 'consteval auto
bio::alphabet::tag_invoke(custom::size, dna4) requires 
constexpr_default_initializable<derived_type>'
/home/hannes/devel/biocpp-core/include/bio/alphabet/base.hpp:260:27:   required
from 'class bio::alphabet::base<bio::alphabet::dna4, 4, char>'
/home/hannes/devel/biocpp-core/include/bio/alphabet/nucleotide/nucleotide_base.hpp:41:7:
  required from 'class bio::alphabet::nucleotide_base<bio::alphabet::dna4, 4>'
/home/hannes/devel/biocpp-core/include/bio/alphabet/nucleotide/dna4.hpp:48:21: 
 required from here
/usr/local/lib/gcc12/include/c++/type_traits:964:52: error: static assertion
failed: template argument must be a complete class or an unbounded array
  964 |      
static_assert(std::__is_complete_or_unbounded(__type_identity<_Tp>{}),
      |                    
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/lib/gcc12/include/c++/type_traits:964:52: note:
'std::__is_complete_or_unbounded<__type_identity<bio::alphabet::dna4>
>((std::__type_identity<bio::alphabet::dna4>(),
std::__type_identity<bio::alphabet::dna4>()))' evaluates to false
test.cpp:3:30: error: static assertion failed
    3 | static_assert(bio::alphabet::alphabet<bio::alphabet::dna4>);
      |               ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test.cpp:3:30: note: constraints not satisfied
In file included from /usr/local/lib/gcc12/include/c++/compare:39,
                 from /usr/local/lib/gcc12/include/c++/bits/stl_pair.h:65:
/usr/local/lib/gcc12/include/c++/concepts:132:13:   required for the
satisfaction of 'destructible<_Tp>' [with _Tp = bio::alphabet::dna4]
/usr/local/lib/gcc12/include/c++/concepts:136:13:   required for the
satisfaction of 'constructible_from<_Tp, _Tp>' [with _Tp = bio::alphabet::dna4]
/usr/local/lib/gcc12/include/c++/concepts:150:13:   required for the
satisfaction of 'move_constructible<_Tp>' [with _Tp = bio::alphabet::dna4]
/usr/local/lib/gcc12/include/c++/concepts:155:13:   required for the
satisfaction of 'copy_constructible<t>' [with t = bio::alphabet::dna4]
/home/hannes/devel/biocpp-core/include/bio/alphabet/concept.hpp:562:9:  
required for the satisfaction of 'semialphabet<t>' [with t =
bio::alphabet::dna4]
/usr/local/lib/gcc12/include/c++/concepts:132:28: note: the expression
'is_nothrow_destructible_v<_Tp> [with _Tp = bio::alphabet::dna4]' evaluated to
'false'
  132 |     concept destructible = is_nothrow_destructible_v<_Tp>;
      |

[Bug c++/110765] [13 regression] constraints on parameters of derived type in CRTP base

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110765

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |DUPLICATE
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |ppalka at gcc dot gnu.org

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Thanks for the bug report.  This looks like a dup of PR109751, which is an
unexpected consequence of the CWG2596 resolution (which we proactively
implement IIUC).

A general workaround is to turn the constrained hidden friend into a template,
so that its constraints don't get immediately checked upon instantiation of its
enclosing class (at which point the derived class is still incomplete).

template<class...>
friend constexpr auto tag_invoke(custom::to_char, derived_type const a)
noexcept
    requires(requires { { a.to_char() }; })
{
    return a.to_char();
}

*** This bug has been marked as a duplicate of bug 109751 ***

[Bug c++/110765] [13 regression] constraints on parameters of derived type in CRTP base

[h2+bugs at fsfe dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110765

--- Comment #2 from Hannes Hauswedell <h2+bugs at fsfe dot org> ---
Thanks for the quick reply. I agree it is the same problem.