PR c++/92446 - deduction of class NTTP.

Another place we need to look through the VIEW_CONVERT_EXPR we add to make a
use of a class NTTP have const type.

	* pt.c (deducible_expression): Look through VIEW_CONVERT_EXPR.

From-SVN: r279228

gcc/cp/ChangeLog

+2019-12-11  Jason Merrill  <jason@redhat.com>
+
+	PR c++/92446 - deduction of class NTTP.
+	* pt.c (deducible_expression): Look through VIEW_CONVERT_EXPR.
+
 2019-12-10  Jason Merrill  <jason@redhat.com>
 
 	PR c++/92847 - C++20 comparison ambiguity with class template.

gcc/cp/pt.c

@@ -21183,7 +21183,7 @@ static bool
 deducible_expression (tree expr)
 {
   /* Strip implicit conversions.  */
-  while (CONVERT_EXPR_P (expr))
+  while (CONVERT_EXPR_P (expr) || TREE_CODE (expr) == VIEW_CONVERT_EXPR)
     expr = TREE_OPERAND (expr, 0);
   return (TREE_CODE (expr) == TEMPLATE_PARM_INDEX);
 }

gcc/testsuite/g++.dg/cpp2a/nontype-class26.C

+// { dg-do compile { target c++2a } }
+
+struct p { unsigned p_ {}; };
+
+template <p i> struct pp {};
+struct qq : public pp <p {}> {};
+
+template <p i> int f (pp <i> const &);
+
+int main ()
+{
+  return f (qq {});
+}