c++: Don't reject calls through PMF during constant evaluation [PR102786]

The following testcase incorrectly rejects the c initializer,
while in the s.*a case cxx_eval_* sees .__pfn reads etc.,
in the s.*&S::foo case get_member_function_from_ptrfunc creates
expressions which use INTEGER_CSTs with type of pointer to METHOD_TYPE.
And cxx_eval_constant_expression rejects any INTEGER_CSTs with pointer
type if they aren't 0.
Either we'd need to make sure we defer such folding till cp_fold but the
function and pfn_from_ptrmemfunc is used from lots of places, or
the following patch just tries to reject only non-zero INTEGER_CSTs
with pointer types if they don't point to METHOD_TYPE in the hope that
all such INTEGER_CSTs with POINTER_TYPE to METHOD_TYPE are result of
folding valid pointer-to-member function expressions.
I don't immediately see how one could create such INTEGER_CSTs otherwise,
cast of integers to PMF is rejected and would have the PMF RECORD_TYPE
anyway, etc.

2021-10-19  Jakub Jelinek  <jakub@redhat.com>

	PR c++/102786
	* constexpr.c (cxx_eval_constant_expression): Don't reject
	INTEGER_CSTs with type POINTER_TYPE to METHOD_TYPE.

	* g++.dg/cpp2a/constexpr-virtual19.C: New test.

(cherry picked from commit f45610a45236e97616726ca042898d6ac46a082e)

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 43929e5c40b4bfaffb6eeb5f64d1146a0b51b211..2725059dd85a30959a1cba3d9aa51e48652468c4 100644 (file)
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -4469,6 +4469,10 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 
       if (TREE_CODE (t) == INTEGER_CST
          && TYPE_PTR_P (TREE_TYPE (t))
+         /* INTEGER_CST with pointer-to-method type is only used
+            for a virtual method in a pointer to member function.
+            Don't reject those.  */
+         && TREE_CODE (TREE_TYPE (TREE_TYPE (t))) != METHOD_TYPE
          && !integer_zerop (t))
        {
          if (!ctx->quiet)

diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual19.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual19.C
new file mode 100644 (file)
index 0000000..a55c2b1
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual19.C
@@ -0,0 +1,11 @@
+// PR c++/102786
+// { dg-do compile { target c++2a } }
+
+struct S {
+  virtual constexpr int foo () const { return 42; }
+};
+
+constexpr S s;
+constexpr auto a = &S::foo;
+constexpr auto b = (s.*a) ();
+constexpr auto c = (s.*&S::foo) ();