Dan Zheng e5cb871428 [AutoDiff upstream] Add flag-gated AdditiveArithmetic derivation. (#30628) ...

Add `AdditiveArithmetic` derived conformances for structs, gated by the
`-enable-experimential-additive-arithmetic-derivation` flag.

Structs whose stored properties all conform to `AdditiveArithmetic` can derive
`AdditiveArithmetic`:
- `static var zero: Self`
- `static func +(lhs: Self, rhs: Self) -> Self`
- `static func -(lhs: Self, rhs: Self) -> Self`
- An "effective memberwise initializer":
  - Either a synthesized memberwise initializer or a user-defined initializer
    with the same type.

Effective memberwise initializers are used only by derived conformances for
`Self`-returning protocol requirements like `AdditiveArithmetic.+`, which
require memberwise initialization.

Resolves TF-844.
Unblocks TF-845: upstream `Differentiable` derived conformances.

2020-03-25 10:31:50 -07:00

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