[SR-13310] Unexpected "class member is not '@differentiable'" diagnostic

[dan-zheng]

Previous ID	SR-13310
Radar	None
Original Reporter	@dan-zheng
Type	Bug

Additional Detail from JIRA

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

md5: 665ce04996a9742f44d55033340d329c

Issue Description:

import _Differentiation

class Class: Differentiable {
  var x: Float = 0
}

@differentiable
func foo(_ c: Class) -> Float {
  c.x
}

$ swiftc class.swift
class.swift:9:5: error: expression is not differentiable
  c.x
    ^
class.swift:9:5: note: member is not differentiable because the corresponding class member is not '@differentiable'
  c.x
    ^

This error is not expected because Differentiable derived conformances are expected to add @differentiable attributes to all "differentiation" stored properties (those used to synthesize TangentVector).