Index: lib/Sema/SemaExpr.cpp
===================================================================
--- lib/Sema/SemaExpr.cpp
+++ lib/Sema/SemaExpr.cpp
@@ -7399,6 +7399,18 @@
         return IncompatibleVectors;
       }
     }
+
+    // When the RHS comes from another lax conversion (e.g. binops between
+    // scalars and vectors) the result is canonicalized as a vector. When the
+    // LHS is also a vector, the lax is allowed by the condition above. Handle
+    // the case where LHS is a scalar.
+    if (LHSType->isScalarType() && isLaxVectorConversion(RHSType, LHSType)) {
+      ExprResult *VecExpr = &RHS;
+      *VecExpr = ImpCastExprToType(VecExpr->get(), LHSType, CK_BitCast);
+      Kind = CK_BitCast;
+      return Compatible;
+    }
+
     return Incompatible;
   }
 
Index: test/Sema/vector-cast.c
===================================================================
--- test/Sema/vector-cast.c
+++ test/Sema/vector-cast.c
@@ -45,17 +45,24 @@
 }
 
 typedef float float2 __attribute__ ((vector_size (8)));
+typedef __attribute__((vector_size(8))) double float64x1_t;
+typedef __attribute__((vector_size(16))) double float64x2_t;
+float64x1_t vget_low_f64(float64x2_t __p0);
 
 void f4() {
   float2 f2;
-  double d;
+  double d, a, b, c;
+  float64x2_t v = {0.0, 1.0};
   f2 += d;
-  // We used to allow the next statement, but we've always rejected the next two
-  // statements
+  a = 3.0 + vget_low_f64(v);
+  b = vget_low_f64(v) + 3.0;
+  c = vget_low_f64(v);
+  // Compound assignments are not supported.
   // FIXME: This diagnostic is inaccurate.
   d += f2; // expected-error {{cannot convert between vector values of different size}}
-  d = f2; // expected-error {{assigning to 'double' from incompatible type 'float2'}}
-  d = d + f2; // expected-error {{assigning to 'double' from incompatible type 'float2'}}
+  c -= vget_low_f64(v); // expected-error {{cannot convert between vector values of different size}}
+  d = f2;
+  d = d + f2;
 }
 
 // rdar://15931426