[Sema] Minimum access for override method is fileprivate

[rintaro]

What's in this pull request?

CC: @jrose-apple
As discussed in #4348 (comment)

Minimum access level of override member in private classes should be fileprivate.
This C.init() and C.foo() is illegal now.

public class B {
  public required init() {}
  public func foo() {}
}
private class C : B {
  private required init() {} // 'required' initializer must be as accessible as its enclosing type
  private override func foo() {} // overriding instance method must be as accessible as its enclosing type
}

---

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[rintaro]

@swift-ci Please test

[swift-ci]

Build failed
Jenkins build - Swift Test Linux Platform
Git Commit - 3e8def2
Test requested by - @rintaro

[rintaro]

https://ci.swift.org/job/swift-PR-Linux/3523/
Foundation needs update

Foundation/Unit.swift:101:27: error: overriding instance method must be as accessible as its enclosing type
06:34:11     private override func baseUnitValue(fromValue value: Double) -> Double {
06:34:11     ~~~~~~~               ^
06:34:11     fileprivate
06:34:11 Foundation/Unit.swift:23:15: note: overridden declaration is here
06:34:11     open func baseUnitValue(fromValue value: Double) -> Double {
06:34:11               ^
06:34:11 Foundation/Unit.swift:105:27: error: overriding instance method must be as accessible as its enclosing type
06:34:11     private override func value(fromBaseUnitValue baseUnitValue: Double) -> Double {
06:34:11     ~~~~~~~               ^
06:34:11     fileprivate
06:34:11 Foundation/Unit.swift:33:15: note: overridden declaration is here
06:34:11     open func value(fromBaseUnitValue baseUnitValue: Double) -> Double {
06:34:11               ^

[jrose-apple]

This is certainly an incremental improvement (once the Foundation change goes in), but really it should probably be switched over to access scopes like #4176. Otherwise we'll do the wrong thing for members that are more public than the enclosing type.

[CodaFi]

@rintaro Foundation patches landed.

@swift-ci please test Linux platform.

[jrose-apple]

Do you think it makes sense to land this or to go all the way to access scopes?

[rintaro]

@jrose-apple
Even while we are implementing https://bugs.swift.org/browse/SR-2209,
We should stop people from writing private override.
Yes, I think It's reasonable to merge this for now.

[jrose-apple]

Ah, we don't have to get to SR-2209 to get the checking like #4176 (which was merged), but okay.

[rintaro]

Hm, sorry I'm not fully understand #4176 or SR-2209.
Anyway, I will take vacation next week. I don't have time to do that.

Thank you anyway!