Remove conformance of Complex to Differentiable

The conformance was breaking CI testing, because the CI bots use a macOS version that predates availability of the _Differentiation module, and the compiler-generated differentation operations have neither availability attributes nor checks, so any reference to those operations simply crashes in dyld because the library is not present. We can add availability, but that's not really accurate either, because these operations are not actually binary-stable, so while we wouldn't crash in dyld, we also might not work correctly in the future. We could mark them unavailable on Apple platforms, but that's also not quite right (and further means that they're not going to get a lot of attention). The best solution seems to be to remove the conformance (which was never source-stable).

Author: stephentyrone

README.md

@@ -59,7 +59,7 @@ To use Swift Numerics in a SwiftPM project:
 ## Source stability
 
 The Swift Numerics package is source stable; version numbers follow [Semantic Versioning](https://semver.org).
-The public API of the `swift-numerics` package consists of non-underscored declarations that are marked either `public` or `usableFromInline` in modules re-exported by the top-level `Numerics` module, *excepting any API that involves a conformance to Differentiable (because Differentiable itself is not stable in Swift)*.
+The public API of the `swift-numerics` package consists of non-underscored declarations that are marked either `public` or `usableFromInline` in modules re-exported by the top-level `Numerics` module.
 Interfaces that aren't part of the public API may continue to change in any release, including patch releases. 
 
 Note that contents of the `_NumericsShims` and `_TestSupport` modules, as well as contents of the `Tests` directory, explicitly are not public API.

Sources/ComplexModule/CMakeLists.txt

@@ -12,7 +12,6 @@ add_library(ComplexModule
   Complex+AdditiveArithmetic.swift
   Complex+AlgebraicField.swift
   Complex+Codable.swift
-  Complex+Differentiable.swift
   Complex+ElementaryFunctions.swift
   Complex+Hashable.swift
   Complex+IntegerLiteral.swift

Sources/ComplexModule/Complex+Differentiable.swift

@@ -10,7 +10,6 @@ See https://swift.org/LICENSE.txt for license information
 add_library(ComplexTests
   ApproximateEqualityTests.swift
   ArithmeticTests.swift
-  DifferentiableTests.swift
   ElementaryFunctionTests.swift
   PropertyTests.swift)
 set_target_properties(ComplexTests PROPERTIES

Tests/ComplexTests/CMakeLists.txt

@@ -161,21 +161,6 @@ var testCases = [
   IntegerUtilitiesTests.DoubleWidthTests.all,
 ]
 
-#if canImport(_Differentiation)
-extension DifferentiableTests {
-  static var all = testCase([
-    ("testComponentGetter", DifferentiableTests.testComponentGetter),
-    ("testInitializer", DifferentiableTests.testInitializer),
-    ("testConjugate",  DifferentiableTests.testConjugate),
-    ("testArithmetics", DifferentiableTests.testArithmetics),
-  ])
-}
-
-testCases += [
-  DifferentiableTests.all
-]
-#endif
-
 XCTMain(testCases)
 
 #endif