Merge pull request #16467 from rudkx/rdar39802797-4.2

Allowing forming WritableKeyPath to read-only value in Swift 3/4.

Author: rudkx

lib/Sema/CSSimplify.cpp

@@ -4036,8 +4036,19 @@ ConstraintSystem::simplifyKeyPathConstraint(Type keyPathTy,
       // get any special power from being formed in certain contexts, such
       // as the ability to assign to `let`s in initialization contexts, so
       // we pass null for the DC to `isSettable` here.)
-      if (!storage->isSettable(nullptr)
-          || !storage->isSetterAccessibleFrom(DC)) {
+      if (!getASTContext().isSwiftVersionAtLeast(5)) {
+        // As a source-compatibility measure, continue to allow
+        // WritableKeyPaths to be formed in the same conditions we did
+        // in previous releases even if we should not be able to set
+        // the value in this context.
+        if (!storage->isSettable(DC)) {
+          // A non-settable component makes the key path read-only, unless
+          // a reference-writable component shows up later.
+          capability = ReadOnly;
+          continue;
+        }
+      } else if (!storage->isSettable(nullptr)
+                 || !storage->isSetterAccessibleFrom(DC)) {
         // A non-settable component makes the key path read-only, unless
         // a reference-writable component shows up later.
         capability = ReadOnly;

test/Constraints/Inputs/keypath.swift

@@ -0,0 +1,3 @@
+class C {
+  fileprivate(set) var i = 0
+}

test/Constraints/keypath.swift

@@ -0,0 +1,13 @@
+// RUN: %target-typecheck-verify-swift %S/Inputs/keypath.swift -primary-file %s
+
+struct S {
+  let i: Int
+
+  init() {
+    let _: WritableKeyPath<S, Int> = \.i // no error for Swift 3/4
+  }
+}
+
+func test() {
+  let _: WritableKeyPath<C, Int> = \.i // no error for Swift 3/4
+}

test/Constraints/keypath_swift_5.swift

@@ -0,0 +1,13 @@
+// RUN: %target-typecheck-verify-swift %S/Inputs/keypath.swift -primary-file %s -swift-version 5
+
+struct S {
+  let i: Int
+
+  init() {
+    let _: WritableKeyPath<S, Int> = \.i // expected-error {{type of expression is ambiguous without more context}}
+  }
+}
+
+func test() {
+  let _: WritableKeyPath<C, Int> = \.i // expected-error {{type of expression is ambiguous without more context}}
+}

test/expr/unary/keypath/keypath.swift

@@ -514,8 +514,9 @@ struct VisibilityTesting {
       toHaveType: Exactly<WritableKeyPath<VisibilityTesting, Int>>.self)
     expect(&yRef,
       toHaveType: Exactly<WritableKeyPath<VisibilityTesting, Int>>.self)
+    // Allow WritableKeyPath for Swift 3/4 only.
     expect(&zRef,
-      toHaveType: Exactly<KeyPath<VisibilityTesting, Int>>.self)
+      toHaveType: Exactly<WritableKeyPath<VisibilityTesting, Int>>.self)
   }
 
   func inPrivateContext() {
@@ -536,8 +537,9 @@ struct VisibilityTesting2 {
     var xRef = \VisibilityTesting.x
     var yRef = \VisibilityTesting.y
     var zRef = \VisibilityTesting.z
+    // Allow WritableKeyPath for Swift 3/4 only.
     expect(&xRef,
-      toHaveType: Exactly<KeyPath<VisibilityTesting, Int>>.self)
+      toHaveType: Exactly<WritableKeyPath<VisibilityTesting, Int>>.self)
     expect(&yRef,
       toHaveType: Exactly<WritableKeyPath<VisibilityTesting, Int>>.self)
     expect(&zRef,