[Bug c++/106650] [C++23] P2280 - Using unknown references in constant expressions

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106650

--- Comment #4 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:99d114c15523e0bfe7a89ef1947f82eb5ff0260b

commit r14-5979-g99d114c15523e0bfe7a89ef1947f82eb5ff0260b
Author: Marek Polacek <polacek@redhat.com>
Date:   Fri Nov 17 14:48:44 2023 -0500

    c++: P2280R4, Using unknown refs in constant expr [PR106650]

    This patch is an attempt to implement (part of?) P2280, Using unknown
    pointers and references in constant expressions.  (Note that R4 seems to
    only allow References to unknown/Accesses via this, but not Pointers to
    unknown.)

    This patch works to the extent that the test case added in [expr.const]
    works as expected, as well as the test in
   
<https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2280r4.html#the-this-pointer>

    Most importantly, the proposal makes this compile:

      template <typename T, size_t N>
      constexpr auto array_size(T (&)[N]) -> size_t {
          return N;
      }

      void check(int const (&param)[3]) {
          constexpr auto s = array_size(param);
          static_assert (s == 3);
      }

    and I think it would be a pity not to have it in GCC 14.

    What still doesn't work is the test in $3.2:

      struct A2 { constexpr int f() { return 0; } };
      struct B2 : virtual A2 {};
      void f2(B2 &b) { constexpr int k = b.f(); }

    where we say
    error: '* & b' is not a constant expression

    This will be fixed in the future.

            PR c++/106650

    gcc/cp/ChangeLog:

            * constexpr.cc (cxx_eval_constant_expression) <case PARM_DECL>:
Allow
            reference to unknown/this as per P2280.
            <case VAR_DECL>: Allow reference to unknown as per P2280.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp0x/constexpr-array-ptr6.C: Remove dg-error.
            * g++.dg/cpp0x/constexpr-ref12.C: Likewise.
            * g++.dg/cpp0x/constexpr-ref2.C: Adjust dg-error.
            * g++.dg/cpp0x/noexcept34.C: Remove dg-error.
            * g++.dg/cpp1y/lambda-generic-const10.C: Likewise.
            * g++.dg/cpp0x/constexpr-ref13.C: New test.
            * g++.dg/cpp1z/constexpr-ref1.C: New test.
            * g++.dg/cpp1z/constexpr-ref2.C: New test.
            * g++.dg/cpp2a/constexpr-ref1.C: New test.