-Wunused and C++17 structured bindings

	* decl.c (poplevel): Don't warn about unused structured bindings,
	only real variables.
	* error.c (dump_simple_decl): Handle structured bindings.
	* expr.c (mark_exp_read): Look through DECL_VALUE_EXPR.

From-SVN: r248399

gcc/cp/ChangeLog

+2017-05-23  Jason Merrill  <jason@redhat.com>
+
+	-Wunused and C++17 structured bindings
+	* decl.c (poplevel): Don't warn about unused structured bindings,
+	only real variables.
+	* error.c (dump_simple_decl): Handle structured bindings.
+	* expr.c (mark_exp_read): Look through DECL_VALUE_EXPR.
+
 2017-05-23  Nathan Sidwell  <nathan@acm.org>
 
 	* cp-tree.h (PUSH_GLOBAL, PUSH_LOCAL, PUSH_USING): Delete.

gcc/cp/decl.c

@@ -656,7 +656,10 @@ poplevel (int keep, int reverse, int functionbody)
 	if (VAR_P (decl)
 	    && (! TREE_USED (decl) || !DECL_READ_P (decl))
 	    && ! DECL_IN_SYSTEM_HEADER (decl)
-	    && DECL_NAME (decl) && ! DECL_ARTIFICIAL (decl)
+	    /* For structured bindings, consider only real variables, not
+	       subobjects.  */
+	    && (DECL_DECOMPOSITION_P (decl) ? !DECL_VALUE_EXPR (decl)
+		: (DECL_NAME (decl) && !DECL_ARTIFICIAL (decl)))
 	    && type != error_mark_node
 	    && (!CLASS_TYPE_P (type)
 		|| !TYPE_HAS_NONTRIVIAL_DESTRUCTOR (type)

gcc/cp/error.c

@@ -992,6 +992,8 @@ dump_simple_decl (cxx_pretty_printer *pp, tree t, tree type, int flags)
       else
 	dump_decl (pp, DECL_NAME (t), flags);
     }
+  else if (DECL_DECOMPOSITION_P (t))
+    pp_string (pp, M_("<structured bindings>"));
   else
     pp_string (pp, M_("<anonymous>"));
   if (flags & TFF_DECL_SPECIFIERS)

gcc/cp/expr.c

@@ -133,6 +133,9 @@ mark_exp_read (tree exp)
   switch (TREE_CODE (exp))
     {
     case VAR_DECL:
+      if (DECL_VALUE_EXPR (exp))
+	mark_exp_read (DECL_VALUE_EXPR (exp));
+      gcc_fallthrough ();
     case PARM_DECL:
       DECL_READ_P (exp) = 1;
       break;

gcc/testsuite/g++.dg/cpp1z/decomp29.C

+// { dg-options "-std=c++17 -Wunused" }
+
+#include <tuple>
+
+struct A { int i,j,k; };
+
+A f();
+
+int z;
+
+int main()
+{
+  {
+    auto [i,j,k] = f();		// { dg-warning "unused" }
+  }
+  {
+    auto [i,j,k] = f();
+    z = i;
+  }
+  {
+    auto [i,j] = std::tuple{1,2}; // { dg-warning "unused" }
+  }
+  // No parallel second test, because in this case i and j are variables rather
+  // than mere bindings, so there isn't a link between them and using i will
+  // not prevent a warning about unused j.
+}