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

[Jason Merrill via Gcc-patches]

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.

Tested x86_64-pc-linux-gnu, applying to trunk.

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

        PR c++/67825
        * constraint.cc (tsubst_valid_expression_requirement): Call
        convert_to_void.
---
 gcc/cp/constraint.cc                          |  5 ++++-
 gcc/testsuite/g++.dg/cpp2a/concepts-pmf1.C    | 22 +++++++++++++++++++
 gcc/testsuite/g++.dg/cpp2a/concepts-pr67825.C |  2 +-
 3 files changed, 27 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-pmf1.C

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index 9c21ce80256..e5308414879 100644
--- a/gcc/cp/constraint.cc
+++ b/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;
 }
 
 
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-pmf1.C 
b/gcc/testsuite/g++.dg/cpp2a/concepts-pmf1.C
new file mode 100644
index 00000000000..30d2b2d4a48
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-pmf1.C
@@ -0,0 +1,22 @@
+// 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);
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-pr67825.C 
b/gcc/testsuite/g++.dg/cpp2a/concepts-pr67825.C
index 95698e99978..fff414b8eb2 100644
--- a/gcc/testsuite/g++.dg/cpp2a/concepts-pr67825.C
+++ b/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;
 }

base-commit: bab8d9625f4cdeaf9bb45e28ab62abe47c3827f9
-- 
2.18.1