[SR-12793] Bad interaction between @differentiable type-checking and Differentiable derived conformances

[dan-zheng]

Previous ID	SR-12793
Radar	None
Original Reporter	@dan-zheng
Type	Bug
Status	Closed
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug
Assignee	@dan-zheng
Priority	Medium

md5: 2982851f9d316dc7f87ad75961ebafac

Issue Description:

import _Differentiation

struct Struct: Differentiable {
  var x: Float

  @differentiable // either this
  func method() -> Float { x }

  @derivative(of: method) // or this is sufficient to trigger error
  func vjpMethod() -> (value: Float, pullback: (Float) -> TangentVector) { fatalError() }

  mutating func move(along direction: TangentVector) {}
}

$ swiftc test.swift
test.swift:12:39: error: reference to invalid associated type 'TangentVector' of type 'Struct'
  mutating func move(along direction: TangentVector) {}
                                      ^

This occurs because @differentiable and @derivative attribute type-checking currently tries to resolve Differentiable.TangentVector type witnesses, which is ad-hoc logic that messes up Differentiable derived conformances.

  // Try to get the `TangentVector` type witness, in case the conformance has
  // not been fully checked.
  Type tanType = conf.getTypeWitnessByName(type, ctx.Id_TangentVector);
  if (tanType.isNull() || tanType->hasError())
    return ProtocolConformanceRef();
  return conf;

Attribute type-checking should be changed to drop TangentVector type witness resolution. This should simplify code and make things less hacky.

[dan-zheng]

Fixed in #31727

The problematic code (forcibly resolving Differentiable.TangentVector type witnesses) has been removed. Derivative function type calculation now returns llvm::Expected, checking type witnesses on-demand.