Dan Zheng bbe86e908d [AutoDiff upstream] Add Differentiable protocol derived conformances. (#30671) ...

Add `AdditiveArithmetic` derived conformances for structs and classes, gated by
the `-enable-experimental-differentiable-programming` flag.

Structs and classes whose stored properties all conform to `Differentiable` can
derive `Differentiable`:

- `associatedtype TangentVector: Differentiable & AdditiveArithmetic`
  - Member `TangentVector` structs are synthesized whose stored properties are
    all `var` stored properties that conform to `Differentiable` and that are
    not `@noDerivative`.
- `mutating func move(along: TangentVector)`

The `@noDerivative` attribute may be declared on stored properties to opt out of
inclusion in synthesized `TangentVector` structs.

Some stored properties cannot be used in `TangentVector` struct synthesis and
are implicitly marked as `@noDerivative`, with a warning:

- `let` stored properties.
  - These cannot be updated by `mutating func move(along: TangentVector)`.
- Non-`Differentiable`-conforming stored properties.

`@noDerivative` also implies `@_semantics("autodiff.nonvarying")`, which is
relevant for differentiable activity analysis.

Add type-checking and SILGen tests.

Resolves TF-845.

2020-03-27 06:40:04 -07:00

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