diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 94c1237480b..b6502ec3727 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,13 @@
+2017-12-15  Jakub Jelinek  <jakub@redhat.com>
+
+	Backported from mainline
+	2017-11-20  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c++/82781
+	* constexpr.c (cxx_eval_vector_conditional_expression): New function.
+	(cxx_eval_constant_expression) <case VEC_COND_EXPR>: Use it instead
+	of cxx_eval_conditional_expression.
+
 2017-10-30  Paolo Carlini  <paolo.carlini@oracle.com>
 
 	PR c++/82085
diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 41734c402ee..327c093a953 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -2044,6 +2044,45 @@ cxx_eval_conditional_expression (const constexpr_ctx *ctx, tree t,
 				       jump_target);
 }
 
+/* Subroutine of cxx_eval_constant_expression.
+   Attempt to evaluate vector condition expressions.  Unlike
+   cxx_eval_conditional_expression, VEC_COND_EXPR acts like a normal
+   ternary arithmetics operation, where all 3 arguments have to be
+   evaluated as constants and then folding computes the result from
+   them.  */
+
+static tree
+cxx_eval_vector_conditional_expression (const constexpr_ctx *ctx, tree t,
+					bool *non_constant_p, bool *overflow_p)
+{
+  tree arg1 = cxx_eval_constant_expression (ctx, TREE_OPERAND (t, 0),
+					    /*lval*/false,
+					    non_constant_p, overflow_p);
+  VERIFY_CONSTANT (arg1);
+  tree arg2 = cxx_eval_constant_expression (ctx, TREE_OPERAND (t, 1),
+					    /*lval*/false,
+					    non_constant_p, overflow_p);
+  VERIFY_CONSTANT (arg2);
+  tree arg3 = cxx_eval_constant_expression (ctx, TREE_OPERAND (t, 2),
+					    /*lval*/false,
+					    non_constant_p, overflow_p);
+  VERIFY_CONSTANT (arg3);
+  location_t loc = EXPR_LOCATION (t);
+  tree type = TREE_TYPE (t);
+  tree r = fold_ternary_loc (loc, VEC_COND_EXPR, type, arg1, arg2, arg3);
+  if (r == NULL_TREE)
+    {
+      if (arg1 == TREE_OPERAND (t, 0)
+	  && arg2 == TREE_OPERAND (t, 1)
+	  && arg3 == TREE_OPERAND (t, 2))
+	r = t;
+      else
+	r = build3_loc (loc, VEC_COND_EXPR, type, arg1, arg2, arg3);
+    }
+  VERIFY_CONSTANT (r);
+  return r;
+}
+
 /* Returns less than, equal to, or greater than zero if KEY is found to be
    less than, to match, or to be greater than the constructor_elt's INDEX.  */
 
@@ -4361,12 +4400,14 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 					      jump_target);
 	  break;
 	}
-      /* FALLTHRU */
-    case VEC_COND_EXPR:
       r = cxx_eval_conditional_expression (ctx, t, lval,
 					   non_constant_p, overflow_p,
 					   jump_target);
       break;
+    case VEC_COND_EXPR:
+      r = cxx_eval_vector_conditional_expression (ctx, t, non_constant_p,
+						  overflow_p);
+      break;
 
     case CONSTRUCTOR:
       if (TREE_CONSTANT (t))
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index d07efb8df61..8b4e034690f 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,11 @@
+2017-12-15  Jakub Jelinek  <jakub@redhat.com>
+
+	Backported from mainline
+	2017-11-20  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c++/82781
+	* g++.dg/ext/constexpr-pr82781.C: New test.
+
 2017-12-15  Richard Biener  <rguenther@suse.de>
 
 	Backport from mainline
diff --git a/gcc/testsuite/g++.dg/ext/constexpr-pr82781.C b/gcc/testsuite/g++.dg/ext/constexpr-pr82781.C
new file mode 100644
index 00000000000..eee0159351c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/constexpr-pr82781.C
@@ -0,0 +1,12 @@
+// PR c++/82781
+// { dg-do compile { target c++11 } }
+
+typedef int V __attribute__ ((vector_size (16)));
+constexpr V b1 = { 0, 1, 10, 20 };
+constexpr V b2 = { 0, 2, 10, 0 };
+constexpr V b3 = b1 == b2;
+
+static_assert (b3[0] == -1, "");
+static_assert (b3[1] == 0, "");
+static_assert (b3[2] == -1, "");
+static_assert (b3[3] == 0, "");