c++: Add warning about missing 'requires'

I noticed that concepts-lambda14.C had two useless requires-expressions:

  static_assert(requires { C<T>; });

always succeeds, because C<T> is always a valid expression for any type,
regardless of whether C is satisfied for a particular type.  Presumably the
user means

  static_assert(requires { requires C<T>; });

to make the C<T> a nested-requirement.  Of course,

  static_assert(C<T>);

is much simpler and means the same thing; this is more relevant in the
middle of a longer requires-expression, such as the bug this warning found
in cmcstl2:

  template<class I>
  META_CONCEPT input_iterator =
	  input_or_output_iterator<I> &&
	  readable<I> &&
	  requires(I& i, const I& ci) {
		  typename iterator_category_t<I>;
		  derived_from<iterator_category_t<I>, input_iterator_tag>;
		  i++;
	  };

where 'requires' is missing before 'derived_from'.

gcc/ChangeLog:

	* doc/invoke.texi: Document -Wmissing-requires.

gcc/c-family/ChangeLog:

	* c.opt: Add -Wmissing-requires.

gcc/cp/ChangeLog:

	* parser.c (cp_parser_simple_requirement): Warn about missing
	requires.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-lambda14.C: Add expected warnings.

1 files changed, 19 insertions, 0 deletions

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index a959c71..797e70b 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -29911,6 +29911,25 @@ cp_parser_simple_requirement (cp_parser *parser)
   if (expr.get_location() == UNKNOWN_LOCATION)
     expr.set_location (start);
 
+  for (tree t = expr; ; )
+    {
+      if (TREE_CODE (t) == TRUTH_ANDIF_EXPR
+	  || TREE_CODE (t) == TRUTH_ORIF_EXPR)
+	{
+	  t = TREE_OPERAND (t, 0);
+	  continue;
+	}
+      if (concept_check_p (t))
+	{
+	  gcc_rich_location richloc (get_start (start));
+	  richloc.add_fixit_insert_before (start, "requires ");
+	  warning_at (&richloc, OPT_Wmissing_requires, "testing "
+		      "if a concept-id is a valid expression; add "
+		      "%<requires%> to check satisfaction");
+	}
+      break;
+    }
+
   return finish_simple_requirement (expr.get_location (), expr);
 }