c++: Refine check for CTAD placeholder [PR99586]

In the below testcase, during finish_compound_literal for A<B{V}>{},
type_uses_auto finds and returns the CTAD placeholder for B{V}, which
tricks us into attempting CTAD on A<B{V}>{} and leads to bogus errors.

AFAICT 'type' will always be a bare 'auto' in the CTAD case so we don't
need to look deeply to find it; checking template_placeholder_p instead
should suffice here.

gcc/cp/ChangeLog:

	PR c++/99586
	* semantics.c (finish_compound_literal): Check
	template_placeholder_p instead of type_uses_auto.

gcc/testsuite/ChangeLog:

	PR c++/99586
	* g++.dg/cpp2a/nontype-class42.C: New test.

1 files changed, 7 insertions, 8 deletions

diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index b02596f..8eaaaef 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -3036,14 +3036,13 @@ finish_compound_literal (tree type, tree compound_literal,
       return error_mark_node;
     }
 
-  if (tree anode = type_uses_auto (type))
-    if (CLASS_PLACEHOLDER_TEMPLATE (anode))
-      {
-	type = do_auto_deduction (type, compound_literal, anode, complain,
-				  adc_variable_type);
-	if (type == error_mark_node)
-	  return error_mark_node;
-      }
+  if (template_placeholder_p (type))
+    {
+      type = do_auto_deduction (type, compound_literal, type, complain,
+				adc_variable_type);
+      if (type == error_mark_node)
+	return error_mark_node;
+    }
 
   /* Used to hold a copy of the compound literal in a template.  */
   tree orig_cl = NULL_TREE;