c++: Fix crash with broken deduction from {} [PR97895]

Unfortunately, the otherwise beautiful

  for (constructor_elt &elt : *CONSTRUCTOR_ELTS (init))

is not immune to an empty constructor, so we have to check
CONSTRUCTOR_ELTS first.

gcc/cp/ChangeLog:

	PR c++/97895
	* pt.c (do_auto_deduction): Don't crash when the constructor has
	zero elements.

gcc/testsuite/ChangeLog:

	PR c++/97895
	* g++.dg/cpp0x/auto54.C: New test.

2 files changed, 17 insertions, 4 deletions

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 1babf83..a1b6631 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -29250,10 +29250,13 @@ do_auto_deduction (tree type, tree init, tree auto_node,
     return error_mark_node;
 
   if (BRACE_ENCLOSED_INITIALIZER_P (init))
-    /* We don't recurse here because we can't deduce from a nested
-       initializer_list.  */
-    for (constructor_elt &elt : *CONSTRUCTOR_ELTS (init))
-      elt.value = resolve_nondeduced_context (elt.value, complain);
+    {
+      /* We don't recurse here because we can't deduce from a nested
+	 initializer_list.  */
+      if (CONSTRUCTOR_ELTS (init))
+	for (constructor_elt &elt : *CONSTRUCTOR_ELTS (init))
+	  elt.value = resolve_nondeduced_context (elt.value, complain);
+    }
   else
     init = resolve_nondeduced_context (init, complain);
 
diff --git a/gcc/testsuite/g++.dg/cpp0x/auto54.C b/gcc/testsuite/g++.dg/cpp0x/auto54.C
new file mode 100644
index 0000000..0c1815a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/auto54.C
@@ -0,0 +1,10 @@
+// PR c++/97895
+// { dg-do compile { target c++11 } }
+
+namespace std {
+  template<typename T> struct initializer_list {
+    const T *ptr;
+    decltype(sizeof 0) n;
+  };
+  auto a = {}; // { dg-error "unable to deduce" }
+}