C++ PATCH for c++/84424, ICE with constexpr and __builtin_shuffle.

[Jason Merrill <jason@redhat.com>]

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We were crashing when checking whether the value being created for v
is constant, because TYPE_FIELDS is invalid for VECTOR_TYPE.  But it's
easy enough to just return false for a partially-initialized vector.

Tested x86_64-pc-linux-gnu, applying to trunk.

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commit 0a07b4d11d758ccbfe11012fcbc7ee6e94f2b3d0
Author: Jason Merrill <jason@redhat.com>
Date:   Thu Feb 22 17:29:54 2018 -0500

            PR c++/84424 - ICE with constexpr and __builtin_shuffle.
    
            * constexpr.c (reduced_constant_expression_p): Handle CONSTRUCTOR of
            VECTOR_TYPE.

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index f095b038d32..47ff90cb055 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -1763,7 +1763,13 @@ reduced_constant_expression_p (tree t)
       /* And we need to handle PTRMEM_CST wrapped in a CONSTRUCTOR.  */
       tree idx, val, field; unsigned HOST_WIDE_INT i;
       if (CONSTRUCTOR_NO_IMPLICIT_ZERO (t))
-	field = next_initializable_field (TYPE_FIELDS (TREE_TYPE (t)));
+	{
+	  if (TREE_CODE (TREE_TYPE (t)) == VECTOR_TYPE)
+	    /* An initialized vector would have a VECTOR_CST.  */
+	    return false;
+	  else
+	    field = next_initializable_field (TYPE_FIELDS (TREE_TYPE (t)));
+	}
       else
 	field = NULL_TREE;
       FOR_EACH_CONSTRUCTOR_ELT (CONSTRUCTOR_ELTS (t), i, idx, val)
diff --git a/gcc/testsuite/g++.dg/ext/vector34.C b/gcc/testsuite/g++.dg/ext/vector34.C
new file mode 100644
index 00000000000..5cca53070b2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/vector34.C
@@ -0,0 +1,8 @@
+// PR c++/84424
+// { dg-do compile { target c++11 } }
+// { dg-options "" }
+
+typedef int vec __attribute__ ((vector_size (2 * sizeof (int))));
+
+constexpr vec u = { 1, 2 };
+constexpr vec v = __builtin_shuffle (v, u); // { dg-error "" }