c++: lambda template in requires [PR105541]

Since the patch for PR103408, the template parameters for the lambda in this
test have level 1 instead of 2, and we were treating null template args as 1
level of arguments, so tsubst_template_parms decided it had nothing to do.
Fixed by distinguishing between <> and no args at all, which is what we have
in our "substitution" in a requires-expression.

	PR c++/105541

gcc/cp/ChangeLog:

	* cp-tree.h (TMPL_ARGS_DEPTH): 0 for null args.
	* parser.cc (cp_parser_enclosed_template_argument_list):
	Use 0-length TREE_VEC for <>.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/lambda-requires1.C: New test.

gcc/cp/cp-tree.h

@@ -3779,11 +3779,13 @@ struct GTY(()) lang_decl {
 
 /* The depth of a template argument vector.  When called directly by
    the parser, we use a TREE_LIST rather than a TREE_VEC to represent
-   template arguments.  In fact, we may even see NULL_TREE if there
-   are no template arguments.  In both of those cases, there is only
-   one level of template arguments.  */
-#define TMPL_ARGS_DEPTH(NODE)					\
-  (TMPL_ARGS_HAVE_MULTIPLE_LEVELS (NODE) ? TREE_VEC_LENGTH (NODE) : 1)
+   template arguments.  In that case, there is only one level of template
+   arguments.  We may even see NULL_TREE if there are 0 levels of
+   template arguments, as in cp_parser_requires_expression.   */
+#define TMPL_ARGS_DEPTH(NODE)						\
+  ((NODE) == NULL_TREE ? 0						\
+   : TMPL_ARGS_HAVE_MULTIPLE_LEVELS (NODE) ? TREE_VEC_LENGTH (NODE)	\
+   : 1)
 
 /* The LEVELth level of the template ARGS.  The outermost level of
    args is level 1, not level 0.  */

gcc/cp/parser.cc

@@ -32067,7 +32067,10 @@ cp_parser_enclosed_template_argument_list (cp_parser* parser)
   /* Parse the template-argument-list itself.  */
   if (cp_lexer_next_token_is (parser->lexer, CPP_GREATER)
       || cp_lexer_next_token_is (parser->lexer, CPP_RSHIFT))
-    arguments = NULL_TREE;
+    {
+      arguments = make_tree_vec (0);
+      SET_NON_DEFAULT_TEMPLATE_ARGS_COUNT (arguments, 0);
+    }
   else
     arguments = cp_parser_template_argument_list (parser);
   /* Look for the `>' that ends the template-argument-list. If we find

gcc/testsuite/g++.dg/cpp2a/lambda-requires1.C

@@ -0,0 +1,4 @@
+// PR c++/105541
+// { dg-do compile { target c++20 } }
+
+static_assert(requires { []<typename T>{}; });