Parent-Task:

• SR-13659 Retroactive derivative registration

blocks:

• TF-1160 Linker error for @usableFromInline derivative but non-@usableFromInline original function

relates to:

• SR-12003 Anonymous func declarations

Issue Description:

Require @derivative functions to have the same access level as their referenced original declaration.
One exception: @derivative functions for a public function can be either public or internal and @usableFromInline.

Same rules should apply to @transpose attribute.

Anonymous func declarations (SR-12003) enable public @derivative functions without publicly exposing the derivative function:

public func bar(_ x: Float) -> Float { x }

// Publicly registered derivative. Derivative function itself is not exposed, however.
@derivative(of: bar)
public func _(_ x: Float) -> (value: Float, differential: (Float) -> Float) {
  (x, { dx in dx })
}

// Publicly registered transpose. Transpose function itself is not exposed, however.
@transpose(of: bar)
public func _(_ v: Float) -> Float { v }