inherit required protocols during TangentVector synthesis

[marcrasi]

We have been running into a limitation with TangentVector synthesis. Here's a proposed solution and implementation.

The Problem

If you have a protocol P that refines Differentiable and that constrains the TangentVector to conform to something that can be derived, the compiler can't derive a conformance to P.

e.g.

protocol P: Differentiable where TangentVector: Encodable {}

struct S: P {
  var x: Float
}

/// error: <Cell 1>:3:8: error: type 'S.TangentVector' does not conform to protocol 'Encodable'
/// struct S: P {
///       ^

Proposed Solution

When deriving the TangentVector, look at the conformance the triggered the derivation, find conformance constraints on TangentVector implied by that conformance, and make TangentVector inherit the required protocols.

One nice property of this solution is that it automatically figures out that TangentVector must conform to AdditiveArithmetic and Differentiable, so these conformances no longer need to be hardcoded in the derivation.

[rxwei]

LGTM. Suggest documenting this behavior in the manifesto and the proposal.

[dan-zheng]

Nice generalization!

To confirm, I wonder if this is a workaround for associated type inference changes in #32578?
Some more context: tensorflow/swift-models#620.

[marcrasi]

To confirm, I wonder if this is a workaround for associated type inference changes in #32578?
Some more context: tensorflow/swift-models#620.

No I think it is unrelated. Those changes look like they affect cases where a protocol constrains the TangentVector to be equal to a specified type. This PR is for when a protocol constrains the TangentVector to conform to something.

[marcrasi]

Suggest documenting this behavior in the manifesto and the proposal.

I added a line to the manifesto in this PR, thanks. I'll add a corresponding line to the proposal in a separate PR because that's in a separate repo.

[marcrasi]

@swift-ci please test