Differentiable conformance derivation now "peers through" property wrappers.
Synthesized TangentVector structs contain wrapped properties' TangentVectors as
stored properties, not wrappers' TangentVectors.
Property wrapper types are not required to conform to `Differentiable`.
Property wrapper types are required to provide `wrappedValue.set`, which is
needed to synthesize `mutating func move(along:)`.
```
import _Differentiation
@propertyWrapper
struct Wrapper<Value> {
var wrappedValue: Value
}
struct Struct: Differentiable {
@Wrapper var x: Float = 0
// Compiler now synthesizes:
// struct TangentVector: Differentiable & AdditiveArithmetic {
// var x: Float
// ...
// }
}
```
Resolves SR-12638.
Previously, two conditions were necessary to enable differentiable programming:
- Using the `-enable-experimental-differentiable-programming` frontend flag.
- Importing the `_Differentiation` module.
Importing the `_Differentiation` module is the true condition because it
contains the required compiler-known `Differentiable` protocol. The frontend
flag is redundant and cumbersome.
Now, the frontend flag is removed.
Importing `_Differentiation` is the only condition.
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.
