[Bug c++/117615] [12/13/14/15 Regression] constexpr failure static_cast of Derived virtual Pointer to Member function since r6-4014-gdcdbc004d531b4

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117615

--- Comment #8 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-12 branch has been updated by Simon Martin
<simartin@gcc.gnu.org>:

https://gcc.gnu.org/g:ae8d9d2b40aa7fd6a455beda38ff1b3c21728c31

commit r12-10847-gae8d9d2b40aa7fd6a455beda38ff1b3c21728c31
Author: Simon Martin <simon@nasilyan.com>
Date:   Tue Dec 3 14:30:43 2024 +0100

    c++: Don't reject pointer to virtual method during constant evaluation
[PR117615]

    We currently reject the following valid code:

    === cut here ===
    struct Base {
        virtual void doit (int v) const {}
    };
    struct Derived : Base {
        void doit (int v) const {}
    };
    using fn_t = void (Base::*)(int) const;
    struct Helper {
        fn_t mFn;
        constexpr Helper (auto && fn) : mFn(static_cast<fn_t>(fn)) {}
    };
    void foo () {
        constexpr Helper h (&Derived::doit);
    }
    === cut here ===

    The problem is that since r6-4014-gdcdbc004d531b4, &Derived::doit is
    represented with an expression with type pointer to method and using an
    INTEGER_CST (here 1), and that cxx_eval_constant_expression rejects any
    such expression with a non-null INTEGER_CST.

    This patch uses the same strategy as r12-4491-gf45610a45236e9 (fix for
    PR c++/102786), and simply lets such expressions go through.

            PR c++/117615

    gcc/cp/ChangeLog:

            * constexpr.cc (cxx_eval_constant_expression): Don't reject
            INTEGER_CSTs with type POINTER_TYPE to METHOD_TYPE.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp2a/constexpr-virtual22.C: New test.

    (cherry picked from commit 72a2380a306a1c3883cb7e4f99253522bc265af0)