c++: Include the constraint parameter mapping in diagnostic constraint contexts

When diagnosing a constraint error, we currently try to print the constraint
inside a diagnostic constraint context with its template arguments substituted
in.  If substitution fails, then we instead just print the dependent form, as in
the test case below:

  .../diagnostic6.C:14:15: error: static assertion failed
     14 | static_assert(E<int>); // { dg-error "static assertion failed|not a class" }
        |               ^~~~~~
  .../diagnostic6.C:14:15: note: constraints not satisfied
  .../diagnostic6.C:4:11:   required for the satisfaction of ‘C<T>’
  .../diagnostic6.C:8:11:   required for the satisfaction of ‘D<typename T::type>’
  .../diagnostic6.C:14:15: error: ‘int’ is not a class, struct, or union type

But printing just the dependent form sometimes makes it difficult to understand
the underlying failure.  In the above example, for instance, there's no
indication of how the template argument 'int' relates to either of the 'T's.

This patch improves the situation by changing these diagnostics to always print
the dependent form of the constraint, and alongside it the (preferably
substituted) constraint parameter mapping.  So with the same test case below we
now get:

  .../diagnostic6.C:14:15: error: static assertion failed
     14 | static_assert(E<int>); // { dg-error "static assertion failed|not a class" }
        |               ^~~~~~
  .../diagnostic6.C:14:15: note: constraints not satisfied
  .../diagnostic6.C:4:11:   required for the satisfaction of ‘C<T>’ [with T = typename T::type]
  .../diagnostic6.C:8:11:   required for the satisfaction of ‘D<typename T::type>’ [with T = int]
  .../diagnostic6.C:14:15: error: ‘int’ is not a class, struct, or union type

This change arguably makes it easier to figure out what's going on whenever a
constraint fails due to substitution creating an invalid type rather than
failing due to the constraint evaluating to false.

gcc/cp/ChangeLog:

	* cxx-pretty-print.c (pp_cxx_parameter_mapping): Make extern.  Move
	the "[with ]" bits to here from ...
	(pp_cxx_atomic_constraint): ... here.
	* cxx-pretty-print.h (pp_cxx_parameter_mapping): Declare.
	* error.c (rebuild_concept_check): Delete.
	(print_concept_check_info): Print the dependent form of the constraint and the
	preferably substituted parameter mapping alongside it.

gcc/testsuite/ChangeLog:

	* g++.dg/concepts/diagnostic6.C: New test.

gcc/cp/ChangeLog

@@ -1,3 +1,13 @@
+2020-03-20  Patrick Palka  <ppalka@redhat.com>
+
+	* cxx-pretty-print.c (pp_cxx_parameter_mapping): Make extern.  Move
+	the "[with ]" bits to here from ...
+	(pp_cxx_atomic_constraint): ... here.
+	* cxx-pretty-print.h (pp_cxx_parameter_mapping): Declare.
+	* error.c (rebuild_concept_check): Delete.
+	(print_concept_check_info): Print the dependent form of the constraint and the
+	preferably substituted parameter mapping alongside it.
+
 2020-03-19  Jason Merrill  <jason@redhat.com>
 
 	PR c++/94175

gcc/cp/cxx-pretty-print.c

@@ -2878,9 +2878,14 @@ pp_cxx_check_constraint (cxx_pretty_printer *pp, tree t)
 /* Output the "[with ...]" clause for a parameter mapping of an atomic
    constraint.   */
 
-static void
+void
 pp_cxx_parameter_mapping (cxx_pretty_printer *pp, tree map)
 {
+  pp_cxx_whitespace (pp);
+  pp_cxx_left_bracket (pp);
+  pp->translate_string ("with");
+  pp_cxx_whitespace (pp);
+
   for (tree p = map; p; p = TREE_CHAIN (p))
     {
       tree parm = TREE_VALUE (p);
@@ -2903,6 +2908,8 @@ pp_cxx_parameter_mapping (cxx_pretty_printer *pp, tree map)
       if (TREE_CHAIN (p) != NULL_TREE)
 	pp_cxx_separate_with (pp, ';');
     }
+
+  pp_cxx_right_bracket (pp);
 }
 
 void
@@ -2914,14 +2921,7 @@ pp_cxx_atomic_constraint (cxx_pretty_printer *pp, tree t)
   /* Emit the parameter mapping.  */
   tree map = ATOMIC_CONSTR_MAP (t);
   if (map && map != error_mark_node)
-    {
-      pp_cxx_whitespace (pp);
-      pp_cxx_left_bracket (pp);
-      pp->translate_string ("with");
-      pp_cxx_whitespace (pp);
-      pp_cxx_parameter_mapping (pp, map);
-      pp_cxx_right_bracket (pp);
-   }
+    pp_cxx_parameter_mapping (pp, map);
 }
 
 void

gcc/cp/cxx-pretty-print.h

@@ -112,5 +112,6 @@ void pp_cxx_conjunction (cxx_pretty_printer *, tree);
 void pp_cxx_disjunction (cxx_pretty_printer *, tree);
 void pp_cxx_constraint (cxx_pretty_printer *, tree);
 void pp_cxx_constrained_type_spec (cxx_pretty_printer *, tree);
+void pp_cxx_parameter_mapping (cxx_pretty_printer *, tree);
 
 #endif /* GCC_CXX_PRETTY_PRINT_H */

gcc/cp/error.c

@@ -3680,27 +3680,6 @@ print_location (diagnostic_context *context, location_t loc)
                  "locus", xloc.file, xloc.line);
 }
 
-/* Instantiate the concept check for the purpose of diagnosing an error.  */
-
-static tree
-rebuild_concept_check (tree expr, tree map, tree args)
-{
-  /* Instantiate the parameter mapping for the template-id.  */
-  map = tsubst_parameter_mapping (map, args, tf_none, NULL_TREE);
-  if (map == error_mark_node)
-    return error_mark_node;
-  args = get_mapped_args (map);
-
-  /* Rebuild the template id using substituted arguments. Substituting
-     directly through the expression will trigger recursive satisfaction,
-     so don't do that.  */
-  tree id = unpack_concept_check (expr);
-  args = tsubst_template_args (TREE_OPERAND (id, 1), args, tf_none, NULL_TREE);
-  if (args == error_mark_node)
-    return error_mark_node;
-  return build_nt (TEMPLATE_ID_EXPR, TREE_OPERAND (id, 0), args);
-}
-
 static void
 print_constrained_decl_info (diagnostic_context *context, tree decl)
 {
@@ -3717,12 +3696,18 @@ print_concept_check_info (diagnostic_context *context, tree expr, tree map, tree
   tree tmpl = TREE_OPERAND (id, 0);
   if (OVL_P (tmpl))
     tmpl = OVL_FIRST (tmpl);
-  tree check = rebuild_concept_check (expr, map, args);
-  if (check == error_mark_node)
-    check = expr;
 
   print_location (context, DECL_SOURCE_LOCATION (tmpl));
-  pp_verbatim (context->printer, "required for the satisfaction of %qE\n", check);
+
+  cxx_pretty_printer *pp = (cxx_pretty_printer *)context->printer;
+  pp_verbatim (pp, "required for the satisfaction of %qE", expr);
+  if (map && map != error_mark_node)
+    {
+      tree subst_map = tsubst_parameter_mapping (map, args, tf_none, NULL_TREE);
+      pp_cxx_parameter_mapping (pp, (subst_map != error_mark_node
+				     ? subst_map : map));
+    }
+  pp_newline (pp);
 }
 
 /* Diagnose the entry point into the satisfaction error. Returns the next

gcc/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2020-03-20  Patrick Palka  <ppalka@redhat.com>
+
+	* g++.dg/concepts/diagnostic6.C: New test.
+
 2020-03-20  Srinath Parvathaneni  <srinath.parvathaneni@arm.com>
 
 	* gcc.target/arm/mve/intrinsics/vabdq_x_f16.c: New test.

gcc/testsuite/g++.dg/concepts/diagnostic6.C

@@ -0,0 +1,14 @@
+// { dg-do compile { target c++2a } }
+
+template<typename T>
+  concept C = requires (T t) { t + 0; };
+// { dg-message "satisfaction of .C<T>. .with T = typename T::type." "" { target *-*-* } .-1 }
+
+template<typename T>
+  concept D = C<T>;
+// { dg-message "satisfaction of .D<typename T::type>. .with T = int." "" { target *-*-* } .-1 }
+
+template<typename T>
+  concept E = D<typename T::type>;
+
+static_assert(E<int>); // { dg-error "static assertion failed|not a class" }