Put back subscript.

Author: bartchr808

stdlib/public/core/SIMDVectorTypes.swift.gyb

@@ -45,9 +45,6 @@ public struct SIMD${n}<Scalar>: SIMD where Scalar: SIMDScalar {
   }
 
   /// Accesses the scalar at the specified position.
-  @differentiable(vjp: _vjpSubscript
-  where Scalar : EuclideanDifferentiable & BinaryFloatingPoint,
-        Scalar.TangentVector : BinaryFloatingPoint)
   public subscript(index: Int) -> Scalar {
     @_transparent get {
       _precondition(indices.contains(index))
@@ -206,8 +203,9 @@ extension SIMD${n}
   where Scalar : EuclideanDifferentiable & BinaryFloatingPoint,
         Scalar.TangentVector : BinaryFloatingPoint {
   @usableFromInline
+  @derivative(of: subscript(index:))
   internal func _vjpSubscript(index: Int)
-  -> (Scalar, (Scalar.TangentVector) -> TangentVector) {
+  -> (value: Scalar, pullback: (Scalar.TangentVector) -> TangentVector) {
     return (self[index], { v in
       var zeros = Self.zero
       zeros[index] = Scalar(v)