Index: lib/Sema/SemaExpr.cpp
===================================================================
--- lib/Sema/SemaExpr.cpp
+++ lib/Sema/SemaExpr.cpp
@@ -5552,9 +5552,14 @@
   // ASTContext::getTypeSize will return the size rounded up to a
   // power of 2, so instead of using that, we need to use the raw
   // element size multiplied by the element count.
-  uint64_t srcEltSize = Context.getTypeSize(srcEltTy);
-  uint64_t destEltSize = Context.getTypeSize(destEltTy);
-  
+  //
+  // We need to be careful about booleans though; they're lowered to i1s, but
+  // getTypeSize views them as i8s.
+  uint64_t srcEltSize =
+      srcEltTy->isBooleanType() ? 1 : Context.getTypeSize(srcEltTy);
+  uint64_t destEltSize =
+      destEltTy->isBooleanType() ? 1 : Context.getTypeSize(destEltTy);
+
   return (srcLen * srcEltSize == destLen * destEltSize);
 }
 
Index: test/CodeGenCXX/bool-vector-conversion.cpp
===================================================================
--- /dev/null
+++ test/CodeGenCXX/bool-vector-conversion.cpp
@@ -0,0 +1,25 @@
+// RUN: %clang_cc1 %s -emit-llvm -o - | FileCheck %s
+//
+// In many parts of clang, we treat a bool as an i8, but we lower it to an i1.
+// This is usually fine, but it broke things with vectors (e.g. we deemed a cast
+// from a [4 x i1] to a [1 x i32] to be legal, and proceeded to crash shortly
+// afterward).
+
+// Nothing but OpenCL allows vectors of booleans.
+// CHECK-LABEL: @_Z4testv
+void test() {
+  typedef __attribute__((__ext_vector_type__(8))) bool CLVectorBool8;
+  typedef __attribute__((__ext_vector_type__(1))) unsigned char CLVectorInt1;
+
+  // CHECK: store <8 x i1> zeroinitializer
+  CLVectorBool8 bools = (CLVectorBool8)false;
+  // CHECK: store <1 x i8>
+  CLVectorInt1 ints = (CLVectorInt1)bools;
+  // CHECK: store <8 x i1>
+  bools = (CLVectorBool8)ints;
+
+  // Run through the code in CGExprConstant.
+  // CHECK: store <8 x i1> zeroinitializer
+  bools = (CLVectorBool8)(CLVectorInt1)(CLVectorBool8)false;
+}
+