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

[Simon Martin]

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.

Successfully tested on x86_64-pc-linux-gnu.

        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.

---
 gcc/cp/constexpr.cc                           | 27 ++++++++++++-------
 .../g++.dg/cpp2a/constexpr-virtual22.C        | 22 +++++++++++++++
 2 files changed, 40 insertions(+), 9 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-virtual22.C

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 5a87fa485c6..d9636bad683 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -8277,15 +8277,24 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, 
tree t,
              }
            else
              {
-               /* This detects for example:
-                    reinterpret_cast<void*>(sizeof 0)
-               */
-               if (!ctx->quiet)
-                 error_at (loc, "%<reinterpret_cast<%T>(%E)%> is not "
-                           "a constant expression",
-                           type, op);
-               *non_constant_p = true;
-               return t;
+               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:
+                        reinterpret_cast<void*>(sizeof 0)
+                   */
+                   if (!ctx->quiet)
+                     error_at (loc, "%<reinterpret_cast<%T>(%E)%> is not "
+                               "a constant expression",
+                               type, op);
+                   *non_constant_p = true;
+                   return t;
+                 }
              }
          }
 
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 00000000000..89330bf8620
--- /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);
+}
-- 
2.44.0