Robert Widmann 75207cbe23 Allow Deriving Type Witnesses Across Files ...

A recent change to witness matching in #32578 suddenly made the
following construction illegal:

// File1.swift
enum MyEnumInAnotherFile { /**/ }

// File2.swift
extension MyEnumInAnotherFile {
  static var allCases: [MyEnumInAnotherFile] { /**/ }
}

Because it was no longer possible to derive the type witness for
`AllCases`. This is because, when inference ran before synthesis, we
would use the value witness to pick out the type witness and thus had no
need for synthesis. Now that we run synthesis first, we ought to just
allow deriving type witnesses across files, but still ban deriving value
witnesses. In general, if you can utter a type in a different file to
extend it, you should be able to see the requirements necessary to
derive a default type witness.

rdar://66279278, rdar://66279375, rdar://66279384, rdar://66279415, rdar://66279503

2020-07-29 12:10:26 -07:00

..

class_differentiable_other_module.swift

Allow Deriving Type Witnesses Across Files

2020-07-29 12:10:26 -07:00

struct_additive_arithmetic_other_module.swift

…

struct_differentiable_other_module.swift

Allow Deriving Type Witnesses Across Files

2020-07-29 12:10:26 -07:00