c++: Fix invalid pointer-to-member in requires [PR67825]

A recent change to cmcstl2 led to two tests failing due to this bug: our
valid expression checking in the context of a requires-expression wasn't
catching that an expression of member function type can only appear as the
function operand of a call expression.  Fixed by using convert_to_void to do
the same checking as a discarded-value expression.

This patch also fixes 67825, which already had a testcase, but the testcase
was testing for the wrong behavior.

gcc/cp/ChangeLog
2020-04-04  Jason Merrill  <jason@redhat.com>

	PR c++/67825
	* constraint.cc (tsubst_valid_expression_requirement): Call
	convert_to_void.

gcc/cp/ChangeLog

 2020-04-04  Jason Merrill  <jason@redhat.com>
 
+	PR c++/67825
+	* constraint.cc (tsubst_valid_expression_requirement): Call
+	convert_to_void.
+
+2020-04-04  Jason Merrill  <jason@redhat.com>
+
 	PR c++/94453
 	* constexpr.c (maybe_constant_value): Use break_out_target_exprs.
 	* expr.c (mark_use) [VIEW_CONVERT_EXPR]: Don't wrap a TARGET_EXPR in

gcc/cp/constraint.cc

@@ -1864,7 +1864,10 @@ hash_placeholder_constraint (tree c)
 static tree
 tsubst_valid_expression_requirement (tree t, tree args, subst_info info)
 {
-  return tsubst_expr (t, args, info.complain, info.in_decl, false);
+  tree r = tsubst_expr (t, args, info.complain, info.in_decl, false);
+  if (convert_to_void (r, ICV_STATEMENT, info.complain) == error_mark_node)
+    return error_mark_node;
+  return r;
 }
 
 

gcc/testsuite/g++.dg/cpp2a/concepts-pmf1.C

+// Make sure that the requirement fails because a .* expression of function
+// type can only be used in a call.
+
+// { dg-do compile { target concepts } }
+
+template<class D, class T>
+constexpr decltype(auto) invoke(D (T::*pmd), T&& t)
+  noexcept(noexcept(t.*pmd))
+  requires requires { t.*pmd; }
+ { return t.*pmd; }
+
+char invoke(...);
+
+struct A
+{
+  int f();
+};
+
+int main()
+{
+  static_assert(sizeof(invoke (&A::f, A())) == 1);
+}

gcc/testsuite/g++.dg/cpp2a/concepts-pr67825.C

@@ -15,6 +15,6 @@ template <class X> concept bool C() {
 }
 
 int main() {
-  static_assert(C<A>());
+  static_assert(!C<A>());
   return 0;
 }