PR c++/78358 - tuple decomposition decltype

	* semantics.c (finish_decltype_type): Strip references for a tuple
	decomposition.
	* cp-tree.h (DECL_DECOMPOSITION_P): False for non-variables.

From-SVN: r242432

gcc/cp/ChangeLog

 2016-11-15  Jason Merrill  <jason@redhat.com>
 
+	PR c++/78358
+	* semantics.c (finish_decltype_type): Strip references for a tuple
+	decomposition.
+	* cp-tree.h (DECL_DECOMPOSITION_P): False for non-variables.
+
 	* decl2.c (decl_maybe_constant_var_p): References qualify.
 	* constexpr.c (non_const_var_error): Handle references.
 	* init.c (constant_value_1): Always check decl_constant_var_p.

gcc/cp/cp-tree.h

@@ -3627,10 +3627,10 @@ more_aggr_init_expr_args_p (const aggr_init_expr_arg_iterator *iter)
   (DECL_LANG_SPECIFIC (VAR_DECL_CHECK (NODE))->u.base.var_declared_inline_p \
    = true)
 
-/* Nonzero if NODE is the artificial VAR_DECL for decomposition
+/* Nonzero if NODE is an artificial VAR_DECL for a C++17 decomposition
    declaration.  */
 #define DECL_DECOMPOSITION_P(NODE) \
-  (DECL_LANG_SPECIFIC (VAR_DECL_CHECK (NODE))			\
+  (VAR_P (NODE) && DECL_LANG_SPECIFIC (NODE)			\
    ? DECL_LANG_SPECIFIC (NODE)->u.base.decomposition_p		\
    : false)
 #define SET_DECL_DECOMPOSITION_P(NODE) \

gcc/cp/semantics.c

@@ -8873,14 +8873,6 @@ finish_decltype_type (tree expr, bool id_expression_or_member_access_p,
       if (identifier_p (expr))
         expr = lookup_name (expr);
 
-      /* The decltype rules for decomposition are different from the rules for
-	 member access; in particular, the decomposition decl gets
-	 cv-qualifiers from the aggregate object, whereas decltype of a member
-	 access expr ignores the object.  */
-      if (VAR_P (expr) && DECL_DECOMPOSITION_P (expr)
-	  && DECL_HAS_VALUE_EXPR_P (expr))
-	return unlowered_expr_type (DECL_VALUE_EXPR (expr));
-
       if (INDIRECT_REF_P (expr))
         /* This can happen when the expression is, e.g., "a.b". Just
            look at the underlying operand.  */
@@ -8898,6 +8890,21 @@ finish_decltype_type (tree expr, bool id_expression_or_member_access_p,
         /* See through BASELINK nodes to the underlying function.  */
         expr = BASELINK_FUNCTIONS (expr);
 
+      /* decltype of a decomposition name drops references in the tuple case
+	 (unlike decltype of a normal variable) and keeps cv-qualifiers from
+	 the containing object in the other cases (unlike decltype of a member
+	 access expression).  */
+      if (DECL_DECOMPOSITION_P (expr))
+	{
+	  if (DECL_HAS_VALUE_EXPR_P (expr))
+	    /* Expr is an array or struct subobject proxy, handle
+	       bit-fields properly.  */
+	    return unlowered_expr_type (expr);
+	  else
+	    /* Expr is a reference variable for the tuple case.  */
+	    return non_reference (TREE_TYPE (expr));
+	}
+
       switch (TREE_CODE (expr))
         {
         case FIELD_DECL:

gcc/testsuite/g++.dg/cpp1z/decomp12.C

+// PR c++/78358
+// { dg-do run }
+// { dg-options -std=c++1z }
+
+#include <tuple>
+
+template <typename, typename> struct same_type;
+template <typename T> struct same_type<T, T> {};
+
+int main() {
+  std::tuple tuple = { 1, 'a', 2.3, true };
+  auto[i, c, d, b] = tuple;
+  same_type<std::tuple_element<0, decltype(tuple)>::type, decltype(i)>{};
+  same_type<decltype(i), int>{};
+  same_type<decltype(c), char>{};
+  same_type<decltype(d), double>{};
+  same_type<decltype(b), bool>{};
+  if (i != 1 || c != 'a' || d != 2.3 || b != true)
+    __builtin_abort ();
+}