[Bug c++/105996] [12/11/12/13 Regression] reinterpret_cast in constexpr failure creating a pair with a function pointer of class parent

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105996

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|reinterpret_cast in         |[12/11/12/13 Regression]
                   |constexpr failure creating  |reinterpret_cast in
                   |a pair with a function      |constexpr failure creating
                   |pointer of class parent     |a pair with a function
                   |                            |pointer of class parent
   Last reconfirmed|                            |2022-06-16
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
      Known to fail|                            |10.3.0, 11.3.0, 12.1.0,
                   |                            |13.0, 8.1.0
                 CC|                            |jakub at gcc dot gnu.org
      Known to work|                            |7.5.0

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Reduced:

template<typename T, typename U> constexpr int make_pair(T&&, U&&) { return 0;
}

struct A {
  void CB() {}
};

struct B : public A {
  void Test() {
  using MyCB = void (B::*)();
  static constexpr auto p = make_pair<int, MyCB>(1, &B::CB);
  }
};


105996.C: In member function 'void B::Test()':
105996.C:10:53: error: a reinterpret_cast is not a constant expression
   static constexpr auto p = make_pair<int, MyCB>(1, &B::CB);
                                                     ^~
Started to fail some time before r256448, probably 


commit e99ef40892b598eeb3fa496cc472e111675d7f6c
Author: Jakub Jelinek 
Date:   Wed Jan 10 16:59:09 2018

    re PR c++/81327 (cast to void* does not suppress -Wclass-memaccess)

            PR c++/81327
            * call.c (maybe_warn_class_memaccess): Add forward declaration.
            Change last argument from tree * to const vec<tree, va_gc> *,
adjust
            args uses and check number of operands too.  Don't strip away any
            nops.  Use maybe_constant_value when looking for INTEGER_CST args.
            Deal with src argument not having pointer type.  Check
            tree_fits_uhwi_p before calling tree_to_uhwi.  Remove useless
            test.
            (build_over_call): Call maybe_warn_class_memaccess here on the
            original arguments.
            (build_cxx_call): Rather than here on converted arguments.

            * g++.dg/Wclass-memaccess-2.C: Don't expect a warning when
explicitly
            cast to void *.

    From-SVN: r256440