[SR-8961] Testcase for more-involved use of @_implements using @_nonoverride.

Author: graydon

test/attr/attr_implements_fp2.swift

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+// RUN: %empty-directory(%t)
+// RUN: echo 'main()' >%t/main.swift
+// RUN: %target-swiftc_driver -o %t/a.out %s %t/main.swift
+// RUN: %target-codesign %t/a.out
+// RUN: %target-run %t/a.out | %FileCheck %s
+// REQUIRES: executable_test
+
+// This is a yet-more-thorough test for rdar://43804798 (and its successor rdar://45183426) that
+// uses @_implements to achieve "Comparable Floating Point values are FP-like when known to be FP,
+// Comparable-like when only known to be comparable".
+//
+// In this variant, the switch in behaviour happens at an intermediate protocol (FauxtingPoint)
+// rather than at concrete types (Fauxt).
+//
+// Unfortunately to make this work, we have to go even further into the weeds and make use of
+// @_nonoverride to provide a protocol-level entry to rank against Comparable.<, because the type
+// system happens to prefer protocol-level entries to extension-level entries when ranking
+// overrides.
+
+// Count calls to the different comparison operators.
+public var comparedAsComparablesCount : Int = 0
+public var comparedAsFauxtingPointsCount : Int = 0
+
+public protocol FauxtingPoint : Comparable {
+  static var nan: Self { get }
+  static var one: Self { get }
+  static var two: Self { get }
+  var bitPattern: UInt8 { get }
+  @_nonoverride static func <(_ lhs: Self, _ rhs: Self) -> Bool
+}
+
+public extension FauxtingPoint {
+  // This version of < will be called in a context that only knows it has a Comparable.
+  @_implements(Comparable, <(_:_:))
+  static func _ComparableLessThan(_ lhs: Self, _ rhs: Self) -> Bool {
+    print("compared as Comparables")
+    comparedAsComparablesCount += 1
+    return lhs.bitPattern < rhs.bitPattern
+  }
+  // This version of < will be called in a context that knows it has FauxtingPoint.
+  static func <(_ lhs: Self, _ rhs: Self) -> Bool {
+    print("compared as FauxtingPoint")
+    comparedAsFauxtingPointsCount += 1
+    if lhs == Self.nan || rhs == Self.nan {
+      return false
+    } else {
+      return lhs.bitPattern < rhs.bitPattern
+    }
+  }
+}
+
+public enum State {
+  case Nan
+  case One
+  case Two
+}
+
+public struct Fauxt {
+  let state: State
+  init(_ s: State) {
+    state = s
+  }
+  public static var nan: Fauxt {
+    return Fauxt(State.Nan)
+  }
+  public static var one: Fauxt {
+    return Fauxt(State.One)
+  }
+  public static var two: Fauxt {
+    return Fauxt(State.Two)
+  }
+}
+
+extension Fauxt: FauxtingPoint {
+  // Requirement from FauxtingPoint
+  public var bitPattern: UInt8 {
+    switch state {
+    case .One:
+      return 1
+    case .Two:
+      return 2
+    case .Nan:
+      return 0xff
+    }
+  }
+}
+
+public extension Fauxt {
+}
+
+public func compare_Comparables<T:Comparable>(_ x: T, _ y: T) -> Bool {
+  return x < y
+}
+
+public func compare_FauxtingPoint<T:FauxtingPoint>(_ x: T, _ y: T) -> Bool {
+  return x < y
+}
+
+public func compare_Fauxts(_ x: Fauxt, _ y: Fauxt) -> Bool {
+  return x < y
+}
+
+public func main() {
+  assert(compare_Comparables(Fauxt.one, Fauxt.two))
+  assert(comparedAsComparablesCount == 1)
+  // CHECK: compared as Comparables
+  assert(compare_Comparables(Fauxt.one, Fauxt.nan))
+  assert(comparedAsComparablesCount == 2)
+  // CHECK: compared as Comparables
+  assert(!compare_Comparables(Fauxt.nan, Fauxt.one))
+  assert(comparedAsComparablesCount == 3)
+  // CHECK: compared as Comparables
+
+  assert(compare_FauxtingPoint(Fauxt.one, Fauxt.two))
+  assert(comparedAsFauxtingPointsCount == 1)
+  // CHECK: compared as FauxtingPoint
+  assert(!compare_FauxtingPoint(Fauxt.one, Fauxt.nan))
+  assert(comparedAsFauxtingPointsCount == 2)
+  // CHECK: compared as FauxtingPoint
+  assert(!compare_FauxtingPoint(Fauxt.nan, Fauxt.one))
+  assert(comparedAsFauxtingPointsCount == 3)
+  // CHECK: compared as FauxtingPoint
+
+  assert(compare_Fauxts(Fauxt.one, Fauxt.two))
+  assert(comparedAsFauxtingPointsCount == 4)
+  // CHECK: compared as FauxtingPoint
+  assert(!compare_Fauxts(Fauxt.one, Fauxt.nan))
+  assert(comparedAsFauxtingPointsCount == 5)
+  // CHECK: compared as FauxtingPoint
+  assert(!compare_Fauxts(Fauxt.nan, Fauxt.one))
+  assert(comparedAsFauxtingPointsCount == 6)
+  // CHECK: compared as FauxtingPoint
+}