PR c++/90098 - partial specialization and class non-type parms.

A non-type template parameter of class type used in an expression has
const-qualified type; the pt.c hunks deal with this difference from the
unqualified type of the parameter declaration.  WAhen we use such a
parameter as an argument to another template, we don't want to confuse
things by copying it, we should pass it straight through.  And we might as
well skip copying other classes in constant evaluation context in a
template, too; we'll get the copy semantics at instantiation time.

	PR c++/90099
	PR c++/90101
	* call.c (build_converted_constant_expr_internal): Don't copy.
	* pt.c (process_partial_specialization): Allow VIEW_CONVERT_EXPR
	around class non-type parameter.
	(unify) [TEMPLATE_PARM_INDEX]: Ignore cv-quals.

From-SVN: r273591

gcc/cp/ChangeLog

+2019-07-18  Jason Merrill  <jason@redhat.com>
+
+	PR c++/90098 - partial specialization and class non-type parms.
+	PR c++/90099
+	PR c++/90101
+	* call.c (build_converted_constant_expr_internal): Don't copy.
+	* pt.c (process_partial_specialization): Allow VIEW_CONVERT_EXPR
+	around class non-type parameter.
+	(unify) [TEMPLATE_PARM_INDEX]: Ignore cv-quals.
+
 2019-07-16  Jason Merrill  <jason@redhat.com>
 
 	* parser.c (make_location): Add overload taking cp_lexer* as last

gcc/cp/call.c

@@ -4278,6 +4278,11 @@ build_converted_constant_expr_internal (tree type, tree expr,
 
   if (conv)
     {
+      /* Don't copy a class in a template.  */
+      if (CLASS_TYPE_P (type) && conv->kind == ck_rvalue
+	  && processing_template_decl)
+	conv = next_conversion (conv);
+
       conv->check_narrowing = true;
       conv->check_narrowing_const_only = true;
       expr = convert_like (conv, expr, complain);

gcc/cp/pt.c

@@ -4954,7 +4954,8 @@ process_partial_specialization (tree decl)
                  simple identifier' condition and also the `specialized
                  non-type argument' bit.  */
               && TREE_CODE (arg) != TEMPLATE_PARM_INDEX
-	      && !(REFERENCE_REF_P (arg)
+	      && !((REFERENCE_REF_P (arg)
+		    || TREE_CODE (arg) == VIEW_CONVERT_EXPR)
 		   && TREE_CODE (TREE_OPERAND (arg, 0)) == TEMPLATE_PARM_INDEX))
             {
               if ((!packed_args && tpd.arg_uses_template_parms[i])
@@ -22371,9 +22372,11 @@ unify (tree tparms, tree targs, tree parm, tree arg, int strict,
 	/* Template-parameter dependent expression.  Just accept it for now.
 	   It will later be processed in convert_template_argument.  */
 	;
-      else if (same_type_p (non_reference (TREE_TYPE (arg)),
-			    non_reference (tparm)))
-	/* OK */;
+      else if (same_type_ignoring_top_level_qualifiers_p
+	       (non_reference (TREE_TYPE (arg)),
+		non_reference (tparm)))
+	/* OK.  Ignore top-level quals here because a class-type template
+	   parameter object is const.  */;
       else if ((strict & UNIFY_ALLOW_INTEGER)
 	       && CP_INTEGRAL_TYPE_P (tparm))
 	/* Convert the ARG to the type of PARM; the deduced non-type

gcc/testsuite/g++.dg/cpp2a/nontype-class18.C

+// PR c++/90101
+// { dg-do compile { target c++2a } }
+
+template<typename List>
+struct A;
+
+template<template<auto...> typename List>
+struct A<List<>> {};
+
+template<template<auto...> typename List, auto V>
+struct A<List<V>> {};
+
+template<auto>
+struct B {};
+
+struct X { int value; };
+A<B<X{1}>> a2;

gcc/testsuite/g++.dg/cpp2a/nontype-class19.C

+// PR c++/90099
+// { dg-do compile { target c++2a } }
+
+struct Unit {
+  int value;
+  // auto operator<=>(const Unit&) = default;
+};
+
+template<Unit U, typename... Ts>
+struct X {};
+
+template<Unit U, typename T, typename... Rest>
+struct X<U, T, Rest...> {};

gcc/testsuite/g++.dg/cpp2a/nontype-class20.C

+// PR c++/90098
+// { dg-do compile { target c++2a } }
+
+struct A {
+  int value;
+  // auto operator<=>(const A&) = default;
+};
+
+template<A... Us>
+struct Z {};
+
+template<A V, A... Rest>
+struct Z<V, Rest...> {};