Richard Wei 76d0648e60 [AutoDiff] [Sema] Include certain 'let' properties in 'Differentiable' derived conformances. (#33700) ...

In `Differentiable` derived conformances, `let` properties are currently treated as if they had `@noDerivative` and excluded from the derived `Differentiable` conformance implementation. This is limiting to properties that have a non-mutating `move(along:)` (e.g. class properties), which can be mathematically treated as differentiable variables.

This patch changes the derived conformances behavior such that `let` properties will be included as differentiable variables if they have a non-mutating `move(along:)`. This unblocks the following code:

```swift
final class Foo: Differentiable {
   let x: ClassStuff // Class type with a non-mutating 'move(along:)'

   // Synthesized code:
   //   struct TangentVector {
   //     var x: ClassStuff.TangentVector
   //   }
   //   ...
   //   func move(along direction: TangentVector) {
   //     x.move(along: direction.x)
   //   }
}
```

Resolves SR-13474 (rdar://67982207).

2020-08-29 21:46:58 -07:00

15 KiB

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