[gcc r12-10847] c++: Don't reject pointer to virtual method during constant evaluation [PR117615]

[Simon Martin via Gcc-cvs]

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

commit r12-10847-gae8d9d2b40aa7fd6a455beda38ff1b3c21728c31
Author: Simon Martin <si...@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)

Diff:
---
 gcc/cp/constexpr.cc                              |  6 ++++++
 gcc/testsuite/g++.dg/cpp2a/constexpr-virtual22.C | 22 ++++++++++++++++++++++
 2 files changed, 28 insertions(+)

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 20abbee3600e..6c8d8ab17f29 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -7353,6 +7353,12 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, 
tree t,
                    return t;
                  }
              }
+           else if (TYPE_PTR_P (type)
+                   && TREE_CODE (TREE_TYPE (type)) == METHOD_TYPE)
+             /* INTEGER_CST with pointer-to-method type is only used
+                for a virtual method in a pointer to member function.
+                Don't reject those.  */
+             ;
            else
              {
                /* This detects for example:
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual22.C 
b/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual22.C
new file mode 100644
index 000000000000..89330bf86200
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual22.C
@@ -0,0 +1,22 @@
+// PR c++/117615
+// { dg-do "compile" { target c++20 } }
+
+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);
+    constexpr Helper h2 (&Base::doit);
+}