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.

2 files changed, 28 insertions, 0 deletions

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 5a87fa4..c1d5401 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -8275,6 +8275,12 @@ 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 0000000..89330bf
--- /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);
+}