[gcc r12-8535] c++: lambda template in requires [PR105541]

[gcc r12-8535] c++: lambda template in requires [PR105541]

[Jason Merrill]

https://gcc.gnu.org/g:b1c8ee2627696717013ebdb1ca3f5f97a76b1cb9

commit r12-8535-gb1c8ee2627696717013ebdb1ca3f5f97a76b1cb9
Author: Jason Merrill <jason@redhat.com>
Date:   Wed May 11 14:53:26 2022 -0400

    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.

Diff:
---
 gcc/cp/cp-tree.h                              | 12 +++++++-----
 gcc/cp/parser.cc                              |  5 ++++-
 gcc/testsuite/g++.dg/cpp2a/lambda-requires1.C |  4 ++++
 3 files changed, 15 insertions(+), 6 deletions(-)

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 97610e319bb..2db847868b6 100644
--- a/gcc/cp/cp-tree.h
+++ b/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.  */
diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 1ec345e3811..ad717f73e3b 100644
--- a/gcc/cp/parser.cc
+++ b/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
diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-requires1.C b/gcc/testsuite/g++.dg/cpp2a/lambda-requires1.C
new file mode 100644
index 00000000000..fdfdaebcf6a
--- /dev/null
+++ b/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>{}; });