c++: Improve dump_decl for standard concepts

This patch improves the pretty printing of standard concept definitions in error
messages.  In particular, standard concepts are now printed qualified whenever
appropriate, and the "concept" specifier is printed only when the
TFF_DECL_SPECIFIERS flag is specified.

In the below test, the first error message changes from
  9:15: error: ‘b’ was not declared in this scope; did you mean ‘concept b’?
to
  9:15: error: ‘b’ was not declared in this scope; did you mean ‘a::b’?

gcc/cp/ChangeLog:

	* error.c (dump_decl) [CONCEPT_DECL]: Use dump_simple_decl.
	(dump_simple_decl): Handle standard concept definitions as well as
	variable concept definitions.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts6.C: New test.

gcc/cp/ChangeLog

+2020-02-12  Patrick Palka  <ppalka@redhat.com>
+
+	* error.c (dump_decl) [CONCEPT_DECL]: Use dump_simple_decl.
+	(dump_simple_decl): Handle standard concept definitions as well as
+	variable concept definitions.
+
 2020-02-10  Jakub Jelinek  <jakub@redhat.com>
 
 	PR other/93641

gcc/cp/error.c

@@ -1037,14 +1037,13 @@ dump_simple_decl (cxx_pretty_printer *pp, tree t, tree type, int flags)
 
   if (flags & TFF_DECL_SPECIFIERS)
     {
-      if (VAR_P (t) && DECL_DECLARED_CONSTEXPR_P (t))
-        {
-	  if (DECL_LANG_SPECIFIC (t) && DECL_DECLARED_CONCEPT_P (t))
-	    pp_cxx_ws_string (pp, "concept");
-	  else
-	    pp_cxx_ws_string (pp, "constexpr");
-	}
-      dump_type_prefix (pp, type, flags & ~TFF_UNQUALIFIED_NAME);
+      if (concept_definition_p (t))
+	pp_cxx_ws_string (pp, "concept");
+      else if (VAR_P (t) && DECL_DECLARED_CONSTEXPR_P (t))
+	pp_cxx_ws_string (pp, "constexpr");
+
+      if (!standard_concept_p (t))
+	dump_type_prefix (pp, type, flags & ~TFF_UNQUALIFIED_NAME);
       pp_maybe_space (pp);
     }
   if (! (flags & TFF_UNQUALIFIED_NAME)
@@ -1296,8 +1295,7 @@ dump_decl (cxx_pretty_printer *pp, tree t, int flags)
       break;
 
     case CONCEPT_DECL:
-      pp_cxx_ws_string (pp, "concept");
-      dump_decl_name (pp, DECL_NAME (t), flags);
+      dump_simple_decl (pp, t, TREE_TYPE (t), flags);
       break;
 
     case WILDCARD_DECL:

gcc/testsuite/ChangeLog

+2020-02-12  Patrick Palka  <ppalka@redhat.com>
+
+	* g++.dg/cpp2a/concepts6.C: New test.
+
 2020-02-10  David Malcolm  <dmalcolm@redhat.com>
 
 	PR analyzer/93350

gcc/testsuite/g++.dg/cpp2a/concepts6.C

+// { dg-do compile { target c++2a } }
+
+namespace a
+{
+  template<typename = int>
+    concept b = true; // { dg-message ".a::b. declared here" }
+}
+
+static_assert(b); // { dg-error "did you mean .a::b." }
+
+namespace c
+{
+  template<typename>
+    concept b = true; // { dg-message "concept c::b." }
+
+  template<typename>
+    concept b = true; // { dg-error "concept c::b." }
+}