[Bug c++/112590] New: structural constexpr class fails to instantiate

[Bug c++/112590] New: structural constexpr class fails to instantiate

[janezz55 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112590

            Bug ID: 112590
           Summary: structural constexpr class fails to instantiate
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: janezz55 at gmail dot com
  Target Milestone: ---

Compiling:

#include <cstddef>

template <auto C> void test() {}

template <std::size_t N>
struct S
{
  int a_[N];
  int* f_;
  constexpr S(): a_{}, f_{a_} {}
};

int main()
{
  test<S<10>{}>();
  return 0;
}

fails with:

prog.cc: In function 'int main()':
prog.cc:18:16: error: no matching function for call to 'test<S<10>()>()'
   18 |   test<S<10>{}>();
      |   ~~~~~~~~~~~~~^~
prog.cc:4:6: note: candidate: 'template<auto C> void test()'
    4 | void test()
      |      ^~~~
prog.cc:4:6: note:   template argument deduction/substitution failed:
prog.cc:18:16: error: 'S<10>{int [10](), ((int*)(&<anonymous>.S<10>::a_))}' is
not a constant expression
   18 |   test<S<10>{}>();
      |   ~~~~~~~~~~~~~^~
prog.cc:18:16: error: 'S<10>()' is not a constant expression because it refers
to an incompletely initialized variable

But everything is initialized. I believe a workaround exists, but I didn't
experiment.

https://wandbox.org/permlink/xA81TIzI5XsXdh57

[Bug c++/112590] structural constexpr class fails to instantiate

[mital at mitalashok dot co.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112590

Mital Ashok <mital at mitalashok dot co.uk> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mital at mitalashok dot co.uk

--- Comment #1 from Mital Ashok <mital at mitalashok dot co.uk> ---
See [temp.arg.nontype]p3
<https://timsong-cpp.github.io/cppwp/n4868/temp.arg.nontype#3>:

> For a non-type template-parameter of reference type, or for each non-static
> data member of reference or pointer type in a non-type template-parameter
> of class type or subobject thereof, the reference or pointer value shall
> not refer to or be the address of (respectively):
>  - A temporary object,
>  - [...]
>  - a subject of one of the above.

And "f_{a_}" is initializing the pointer as a subobject of a temporary
object, so this is invalid. Though the error given might be wrong or
misleading.

[Bug c++/112590] structural constexpr class fails to instantiate

[janezz55 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112590

--- Comment #2 from Janez Zemva <janezz55 at gmail dot com> ---
Very nice, but if I write:

int main()
{
  static constexpr S<10> s;
  return 0;
}

there will still be a compile error.

[Bug c++/112590] structural constexpr class fails to instantiate

[janezz55 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112590

--- Comment #3 from Janez Zemva <janezz55 at gmail dot com> ---
Sorry for my last comment. I have prepared a more involved example:

https://github.com/user1095108/ca/blob/master/consttests.cpp

If

test<a>();

is commented out, everything compiles, otherwise not.