[C++ PATCH] Fix structured binding initializer checking (PR c++/81888)

[Jakub Jelinek <jakub@redhat.com>]

Hi!

My PR81258 fix actually rejects even valid cases.
The standard says that:
"The initializer shall be of the form “= assignment-expression”, of the form
“{ assignment-expression }”, or of the form “( assignment-expression )”
Now, if the form is = assigment-expression, we can e.g. in templates end up
with CONSTRUCTOR initializer which has more or fewer elements than 1.

So, this patch restricts the checks to only BRACE_ENCLOSED_INITIALIZER_P
and only if is_direct_init (i.e. not the = assignment-expression form).

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk
(and 7.x after a while)?

2017-11-23  Jakub Jelinek  <jakub@redhat.com>

	PR c++/81888
	* parser.c (cp_parser_decomposition_declaration): Reject just
	BRACE_ENCLOSED_INITIALIZER_P initializers with nelts != 1 rather
	than all such CONSTRUCTORs, and only if is_direct_init is true.

	* g++.dg/cpp1z/decomp30.C: Add a test for structured binding with
	= {} and = { a, a } initializers.
	* g++.dg/cpp1z/decomp31.C: New test.

--- gcc/cp/parser.c.jj	2017-11-21 20:23:01.000000000 +0100
+++ gcc/cp/parser.c	2017-11-23 15:31:44.473524252 +0100
@@ -13382,7 +13382,8 @@ cp_parser_decomposition_declaration (cp_
       if (initializer == NULL_TREE
 	  || (TREE_CODE (initializer) == TREE_LIST
 	      && TREE_CHAIN (initializer))
-	  || (TREE_CODE (initializer) == CONSTRUCTOR
+	  || (is_direct_init
+	      && BRACE_ENCLOSED_INITIALIZER_P (initializer)
 	      && CONSTRUCTOR_NELTS (initializer) != 1))
 	{
 	  error_at (loc, "invalid initializer for structured binding "
--- gcc/testsuite/g++.dg/cpp1z/decomp30.C.jj	2017-09-15 18:11:04.000000000 +0200
+++ gcc/testsuite/g++.dg/cpp1z/decomp30.C	2017-11-23 15:33:04.208552682 +0100
@@ -10,3 +10,5 @@ auto [j, k] { a, a };	// { dg-error "inv
 auto [l, m] = { a };	// { dg-error "deducing from brace-enclosed initializer list requires" }
 auto [n, o] {};		// { dg-error "invalid initializer for structured binding declaration" }
 auto [p, q] ();		// { dg-error "invalid initializer for structured binding declaration" }
+auto [r, s] = {};	// { dg-error "deducing from brace-enclosed initializer list requires" }
+auto [t, u] = { a, a };	// { dg-error "deducing from brace-enclosed initializer list requires" }
--- gcc/testsuite/g++.dg/cpp1z/decomp31.C.jj	2017-11-23 15:22:31.695255014 +0100
+++ gcc/testsuite/g++.dg/cpp1z/decomp31.C	2017-11-23 15:22:21.000000000 +0100
@@ -0,0 +1,18 @@
+// PR c++/81888
+// { dg-do compile { target c++17 } }
+
+struct S {
+  bool s = true;
+};
+
+auto [a] = S{};
+
+template <class T>
+bool
+foo () noexcept
+{
+  auto [c] = T{};
+  return c;
+}
+
+const bool b = foo<S> ();

	Jakub