[stdlib] Workaround type checker ambiguity in Comparable SwiftNewtypeWrapper

[rintaro]

In macOS,

import Foundation
_ = NSLocale.Key.calendar <= NSLocale.Key.countryCode

didn't typecheck because of ambiguity.

This PR adds an additional constraint to Comparable functions for _SwiftNewTypeWrapper,
just like func != for Equatable, RawRepresentable.

[rintaro]

@DougGregor
This fix seems reasonable?
Where and how should I add test cases? I tried test/IDE/newtype.swift, but -verify couldn't catch this because the error does not belong to that file.

@parkera
This is related to swiftlang/swift-corelibs-foundation#829 (comment)

[airspeedswift]

Did it used to type check but has regressed?

[rintaro]

@airspeedswift
I think : _SwiftNewTypeWrapper, Comparable types never worked as Comparable; not a regression.

func acceptComparable<T : Comparable>(_: T) {}
acceptComparable(NSLocale.Key.calendar)
// <unknown>:0: error: type 'NSLocale.Key' does not conform to protocol 'Comparable'
// Swift.Comparable:144:24: note: multiple matching functions named '<=' with type '(NSLocale.Key, NSLocale.Key) -> Bool'

[DougGregor]

This looks like a good change to me. You can add a test to test/ClangImporter/objc_parse.swift

[rintaro]

@DougGregor Thank you!

I added test/ClangImporter/newtype_conformance.swift.
Since these errors are not reported in this file, -verify mode ignores them.
So we can't use -verify for this test case.

<unknown>:0: error: type 'NSNotification.Name' does not conform to protocol 'Comparable'
Swift.Comparable:144:24: note: multiple matching functions named '<=' with type '(NSNotification.Name, NSNotification.Name) -> Bool'
    public static func <=(lhs: Self, rhs: Self) -> Bool
                       ^
Swift.<=:10:13: note: candidate exactly matches
public func <=<T>(lhs: T, rhs: T) -> Bool where T : Comparable
            ^
Swift.<=:1:13: note: candidate exactly matches
public func <=<T>(lhs: T, rhs: T) -> Bool where T : _SwiftNewtypeWrapper, T.RawValue : Comparable
            ^
Swift.Comparable:151:24: note: multiple matching functions named '>=' with type '(NSNotification.Name, NSNotification.Name) -> Bool'
    public static func >=(lhs: Self, rhs: Self) -> Bool
                       ^
Swift.>=:12:13: note: candidate exactly matches
public func >=<T>(lhs: T, rhs: T) -> Bool where T : Comparable
            ^
Swift.>=:1:13: note: candidate exactly matches
public func >=<T>(lhs: T, rhs: T) -> Bool where T : _SwiftNewtypeWrapper, T.RawValue : Comparable
            ^
Swift.Comparable:158:24: note: multiple matching functions named '>' with type '(NSNotification.Name, NSNotification.Name) -> Bool'
    public static func >(lhs: Self, rhs: Self) -> Bool
                       ^
Swift.>:10:13: note: candidate exactly matches
public func ><T>(lhs: T, rhs: T) -> Bool where T : Comparable
            ^
Swift.>:1:13: note: candidate exactly matches
public func ><T>(lhs: T, rhs: T) -> Bool where T : _SwiftNewtypeWrapper, T.RawValue : Comparable
            ^

[DougGregor]

@swift-ci please smoke test and merge

[DougGregor]

@swift-ci please smoke test

[DougGregor]

@swift-ci please smoke test