c++: handle misspelled concepts and missing #include <concepts>

gcc/cp/ChangeLog:
	* name-lookup.cc (suggest_alternative_in_explicit_scope):
	Gracefully handle non-namespaces, such as scoped enums.
	* parser.cc (cp_parser_name_lookup_error): Provide
	a name_hint for the case where we're in an explicit scope.
	* std-name-hint.gperf: Add <concepts>.
	* std-name-hint.h: Regenerate.

gcc/testsuite/ChangeLog:
	* g++.dg/concepts/missing-header.C: New test.
	* g++.dg/concepts/misspelled-concept.C: New test.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>

gcc/cp/name-lookup.cc

@@ -7351,6 +7351,9 @@ suggest_alternative_in_explicit_scope (location_t location, tree name,
   if (name == error_mark_node)
     return name_hint ();
 
+  if (TREE_CODE (scope) != NAMESPACE_DECL)
+    return name_hint ();
+
   /* Resolve any namespace aliases.  */
   scope = ORIGINAL_NAMESPACE (scope);
 

gcc/cp/parser.cc

@@ -3387,8 +3387,25 @@ cp_parser_name_lookup_error (cp_parser* parser,
   if (decl == error_mark_node)
     {
       if (parser->scope && parser->scope != global_namespace)
-	error_at (location, "%<%E::%E%> has not been declared",
-		  parser->scope, name);
+	{
+	  auto_diagnostic_group sentinel;
+	  name_hint hint
+	    = suggest_alternative_in_explicit_scope (location, name,
+						     parser->scope);;
+	  if (const char *suggestion = hint.suggestion ())
+	    {
+	      /* Ideally we'd have a fix-it hint here, but LOCATION
+		 isn't necessarily that of the name.  */
+	      error_at (location,
+			"%<%E::%E%> has not been declared;"
+			" did you mean %<%E::%s%>?",
+			parser->scope, name,
+			parser->scope, suggestion);
+	    }
+	  else
+	    error_at (location, "%<%E::%E%> has not been declared",
+		      parser->scope, name);
+	}
       else if (parser->scope == global_namespace)
 	error_at (location, "%<::%E%> has not been declared", name);
       else if (parser->object_scope

gcc/cp/std-name-hint.gperf

@@ -102,6 +102,39 @@ weak_ordering, "<compare>", cxx20
 # <complex>
 complex, "<complex>", cxx98
 complex_literals, "<complex>", cxx14
+# <concepts>
+same_as, "<concepts>", cxx20
+derived_from, "<concepts>", cxx20
+convertible_to, "<concepts>", cxx20
+common_reference_with, "<concepts>", cxx20
+common_with, "<concepts>", cxx20
+integral, "<concepts>", cxx20
+signed_integral, "<concepts>", cxx20
+unsigned_integral, "<concepts>", cxx20
+floating_point, "<concepts>", cxx20
+assignable_from, "<concepts>", cxx20
+swappable, "<concepts>", cxx20
+swappable_with, "<concepts>", cxx20
+destructible, "<concepts>", cxx20
+constructible_from, "<concepts>", cxx20
+default_initializable, "<concepts>", cxx20
+move_constructible, "<concepts>", cxx20
+copy_constructible, "<concepts>", cxx20
+equality_comparable, "<concepts>", cxx20
+equality_comparable_with, "<concepts>", cxx20
+totally_ordered, "<concepts>", cxx20
+totally_ordered_with, "<concepts>", cxx20
+movable, "<concepts>", cxx20
+copyable, "<concepts>", cxx20
+semiregular, "<concepts>", cxx20
+regular, "<concepts>", cxx20
+invocable, "<concepts>", cxx20
+regular_invocable, "<concepts>", cxx20
+predicate, "<concepts>", cxx20
+relation, "<concepts>", cxx20
+equivalence_relation, "<concepts>", cxx20
+strict_weak_order, "<concepts>", cxx20
+ranges::swap, "<concepts>", cxx20
 # <condition_variable>
 condition_variable, "<condition_variable>", cxx11
 condition_variable_any, "<condition_variable>", cxx11

gcc/testsuite/g++.dg/concepts/missing-header.C

@@ -0,0 +1,6 @@
+// { dg-do compile { target c++20 } }
+// { dg-options "-fconcepts" }
+
+template <std::integral N> foo (N); // { dg-error "'std::integral' has not been declared" }
+// { dg-message "'std::integral' is defined in header '<concepts>'" "note" { target *-*-* } .-1 }
+// { dg-error "expected" "followup" { target *-*-* } .-2 }

gcc/testsuite/g++.dg/concepts/misspelled-concept.C

@@ -0,0 +1,7 @@
+// { dg-do compile { target c++20 } }
+// { dg-options "-fconcepts" }
+
+#include <concepts>
+
+template <std::unsinged_integral N> foo (N); // { dg-error "'std::unsinged_integral' has not been declared; did you mean 'std::unsigned_integral'" }
+// { dg-error "expected" "followup" { target *-*-* } .-1 }