From cf25e27faef75e265e659f39ef6b7d0f1695dfeb Mon Sep 17 00:00:00 2001
From: Patrick Palka <ppalka@redhat.com>
Date: Fri, 2 Apr 2021 19:47:09 -0400
Subject: [PATCH] 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.
---
 gcc/cp/semantics.c                           | 15 +++++++--------
 gcc/testsuite/g++.dg/cpp2a/nontype-class42.C |  8 ++++++++
 2 files changed, 15 insertions(+), 8 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/nontype-class42.C

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;
diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-class42.C b/gcc/testsuite/g++.dg/cpp2a/nontype-class42.C
new file mode 100644
index 0000000..c2e04b2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/nontype-class42.C
@@ -0,0 +1,8 @@
+// PR c++/99586
+// { dg-do compile { target c++20 } }
+
+template <class T>
+struct B { constexpr B(T); };
+
+template <auto> struct A{};
+template <auto V> auto a = A<B{V}>{};
-- 
2.7.4