c++: CTAD within template argument [PR102933]

Here when checking for erroneous occurrences of 'auto' inside a template
argument (which is allowed by the concepts TS for class templates),
extract_autos_r picks up the CTAD placeholder for X{T{0}} which causes
check_auto_in_tmpl_args to reject this valid template argument.  This
patch fixes this by making extract_autos_r ignore CTAD placeholders.

However, it seems we don't need to call check_auto_in_tmpl_args at all
outside of the concepts TS since using 'auto' as a type-id is otherwise
rejected more generally at parse time.  So this patch makes the function
just exit early if !flag_concepts_ts.

Similarly, I think the concepts code paths in do_auto_deduction and
type_uses_auto are only necessary for the concepts TS, so this patch
also restricts these code paths accordingly.

	PR c++/102933

gcc/cp/ChangeLog:

	* parser.c (cp_parser_simple_type_specifier): Adjust diagnostic
	for using auto in parameter declaration.
	* pt.c (extract_autos_r): Ignore CTAD placeholders.
	(extract_autos): Use range-based for.
	(do_auto_deduction): Use extract_autos only for the concepts TS
	and not also for standard concepts.
	(type_uses_auto): Likewise with for_each_template_parm.
	(check_auto_in_tmpl_args): Just return false outside of the
	concepts TS.  Simplify.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/nontype-class50.C: New test.
	* g++.dg/cpp2a/nontype-class50a.C: New test.

4 files changed, 32 insertions, 12 deletions

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index 93335c8..4c20757 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -19513,7 +19513,7 @@ cp_parser_simple_type_specifier (cp_parser* parser,
 	  else if (!flag_concepts)
 	    pedwarn (token->location, 0,
 		     "use of %<auto%> in parameter declaration "
-		     "only available with %<-fconcepts-ts%>");
+		     "only available with %<-std=c++20%> or %<-fconcepts%>");
 	}
       else
 	type = make_auto ();
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 287cf4c..6604003 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -28560,7 +28560,7 @@ static int
 extract_autos_r (tree t, void *data)
 {
   hash_table<auto_hash> &hash = *(hash_table<auto_hash>*)data;
-  if (is_auto (t))
+  if (is_auto (t) && !template_placeholder_p (t))
     {
       /* All the autos were built with index 0; fix that up now.  */
       tree *p = hash.find_slot (t, INSERT);
@@ -28594,10 +28594,8 @@ extract_autos (tree type)
   for_each_template_parm (type, extract_autos_r, &hash, &visited, true);
 
   tree tree_vec = make_tree_vec (hash.elements());
-  for (hash_table<auto_hash>::iterator iter = hash.begin();
-       iter != hash.end(); ++iter)
+  for (tree elt : hash)
     {
-      tree elt = *iter;
       unsigned i = TEMPLATE_PARM_IDX (TEMPLATE_TYPE_PARM_INDEX (elt));
       TREE_VEC_ELT (tree_vec, i)
 	= build_tree_list (NULL_TREE, TYPE_NAME (elt));
@@ -29837,7 +29835,7 @@ do_auto_deduction (tree type, tree init, tree auto_node,
       tree parms = build_tree_list (NULL_TREE, type);
       tree tparms;
 
-      if (flag_concepts)
+      if (flag_concepts_ts)
 	tparms = extract_autos (type);
       else
 	{
@@ -30025,7 +30023,7 @@ type_uses_auto (tree type)
 {
   if (type == NULL_TREE)
     return NULL_TREE;
-  else if (flag_concepts)
+  else if (flag_concepts_ts)
     {
       /* The Concepts TS allows multiple autos in one type-specifier; just
 	 return the first one we find, do_auto_deduction will collect all of
@@ -30048,6 +30046,11 @@ type_uses_auto (tree type)
 bool
 check_auto_in_tmpl_args (tree tmpl, tree args)
 {
+  if (!flag_concepts_ts)
+    /* Only the concepts TS allows 'auto' as a type-id; it'd otherwise
+       have already been rejected by the parser more generally.  */
+    return false;
+
   /* If there were previous errors, nevermind.  */
   if (!args || TREE_CODE (args) != TREE_VEC)
     return false;
@@ -30057,11 +30060,10 @@ check_auto_in_tmpl_args (tree tmpl, tree args)
      We'll only be able to tell during template substitution, so we
      expect to be called again then.  If concepts are enabled and we
      know we have a type, we're ok.  */
-  if (flag_concepts
-      && (identifier_p (tmpl)
-	  || (DECL_P (tmpl)
-	      &&  (DECL_TYPE_TEMPLATE_P (tmpl)
-		   || DECL_TEMPLATE_TEMPLATE_PARM_P (tmpl)))))
+  if (identifier_p (tmpl)
+      || (DECL_P (tmpl)
+	  &&  (DECL_TYPE_TEMPLATE_P (tmpl)
+	       || DECL_TEMPLATE_TEMPLATE_PARM_P (tmpl))))
     return false;
 
   /* Quickly search for any occurrences of auto; usually there won't
diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-class50.C b/gcc/testsuite/g++.dg/cpp2a/nontype-class50.C
new file mode 100644
index 0000000..1c2786a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/nontype-class50.C
@@ -0,0 +1,13 @@
+// PR c++/102933
+// { dg-do compile { target c++20 } }
+
+template<class T> struct X { T t; };
+
+template<X> void f();
+
+template<class T>
+void g() {
+  f<X{T{0}}>();
+}
+
+template void g<int>();
diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-class50a.C b/gcc/testsuite/g++.dg/cpp2a/nontype-class50a.C
new file mode 100644
index 0000000..eb8a6ad
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/nontype-class50a.C
@@ -0,0 +1,5 @@
+// PR c++/102933
+// { dg-do compile { target c++20 } }
+// { dg-additional-options "-fconcepts-ts" }
+
+#include "nontype-class50.C"