c++: Constrained inherited constructor template [PR94549]

A comment in satisfy_declaration_constraints says

  /* For inherited constructors, consider the original declaration;
     it has the correct template information attached. */
  d = strip_inheriting_ctors (d);

but it looks like this comment is wrong when the inherited constructor is for an
instantiation of a constructor template.  In that case, DECL_TEMPLATE_INFO is
correct and DECL_INHERITED_CTOR points to the constructor template of the base
class rather than to the particular instantiation of the constructor template
(and so the DECL_TI_ARGS of the DECL_INHERITED_CTOR are in their dependent
form).

So doing strip_inheriting_ctors in this case then eventually leads to
satisfy_associated_constraints returning true regardless of the constraints
themselves, due to the passed in 'args' being dependent.

An inherited constructor seems to have a non-empty DECL_TEMPLATE_INFO only when
it's for an instantiation of a constructor template, so this patch fixes this
issue by checking for empty DECL_TEMPLATE_INFO before calling
strip_inheriting_ctors.

There is another unguarded call to strip_inheriting_ctors in
get_normalized_constraints_from_decl, but this one seems to be safe to do
unconditionally because the rest of that function doesn't need/look at the
DECL_TI_ARGS of the decl.

gcc/cp/ChangeLog:

	PR c++/94549
	* constraint.cc (satisfy_declaration_constraints): Don't strip the
	inherited constructor if it already has template information.

gcc/testsuite/ChangeLog:

	PR c++/94549
	* g++.dg/concepts/inherit-ctor3.C: Adjust expected diagnostics.
	* g++.dg/cpp2a/concepts-inherit-ctor4.C: New test.
	* g++.dg/cpp2a/concepts-inherit-ctor8.C: New test.

gcc/cp/ChangeLog

 2020-04-21  Patrick Palka  <ppalka@redhat.com>
 
+	PR c++/94549
+	* constraint.cc (satisfy_declaration_constraints): Don't strip the
+	inherited constructor if it already has template information.
+
 	PR c++/94597
 	* pt.c (any_template_parm_r) <case IDENTIFIER_NODE>: New case.  If this
 	is a conversion operator, visit its TREE_TYPE.

gcc/cp/constraint.cc

@@ -2737,9 +2737,10 @@ satisfy_declaration_constraints (tree t, subst_info info)
 {
   gcc_assert (DECL_P (t));
 
-  /* For inherited constructors, consider the original declaration;
-     it has the correct template information attached. */
-  if (flag_new_inheriting_ctors)
+  if (!DECL_TEMPLATE_INFO (t))
+    /* For inherited constructors without template information, consider
+       the original declaration; it has the correct template information
+       attached.  */
     t = strip_inheriting_ctors (t);
 
   /* Update the declaration for diagnostics.  */

gcc/testsuite/ChangeLog

+2020-04-21  Patrick Palka  <ppalka@redhat.com>
+
+	PR c++/94549
+	* g++.dg/concepts/inherit-ctor3.C: Adjust expected diagnostics.
+	* g++.dg/cpp2a/concepts-inherit-ctor4.C: New test.
+	* g++.dg/cpp2a/concepts-inherit-ctor8.C: New test.
+
 2020-04-21  Jonathan Wakely  <jwakely@redhat.com>
 
 	PR c++/94149

gcc/testsuite/g++.dg/concepts/inherit-ctor3.C

@@ -12,12 +12,12 @@ template<typename T>
 
 template<typename T>
   struct S2 : S1<T> { // { dg-error "no matching function" }
-    using S1<T>::S1; // { dg-error "no matching function" }
+    using S1<T>::S1;
   };
 
 struct X { } x;
 
 int main() {
-  S2<X> s1(0); // { dg-error "use of deleted function" }
+  S2<X> s1(0); // { dg-error "no matching function" }
   S2<X> s2; // { dg-error "use of deleted function" }
 }

gcc/testsuite/g++.dg/cpp2a/concepts-inherit-ctor4.C

@@ -10,9 +10,9 @@ template<typename T>
 
 template<typename T>
   struct S2 : S1<T> {
-    using S1<T>::S1; // { dg-error "no matching function" }
+    using S1<T>::S1;
   };
 
 int main() {
-  S2<int> s(0); // { dg-error "use of deleted function" }
+  S2<int> s(0); // { dg-error "no matching function" }
 }

gcc/testsuite/g++.dg/cpp2a/concepts-inherit-ctor8.C

+// PR c++/94549
+// { dg-do compile { target concepts } }
+
+struct base {
+  template <typename type>
+    requires false
+  base(type);
+
+  template <typename type>
+    requires true
+  base(type);
+};
+
+struct derived : base {
+  using base::base;
+};
+
+void foo() {
+  derived{'G'};
+}