[TypeChecker] TypeChecker::isSubtypeOf should recognize Sendable s…

lib/Sema/TypeCheckConstraints.cpp

@@ -1034,8 +1034,22 @@ bool TypeChecker::typesSatisfyConstraint(Type type1, Type type2,
   }
 
   if (auto solution = cs.solveSingle()) {
+    const auto &score = solution->getFixedScore();
     if (unwrappedIUO)
-      *unwrappedIUO = solution->getFixedScore().Data[SK_ForceUnchecked] > 0;
+      *unwrappedIUO = score.Data[SK_ForceUnchecked] > 0;
+
+    // Make sure that Sendable vs. no-Sendable mismatches are
+    // failures here to establish subtyping relationship
+    // (unlike in the solver where they are warnings until Swift 6).
+    if (kind == ConstraintKind::Subtype) {
+      if (score.Data[SK_MissingSynthesizableConformance] > 0)
+        return false;
+
+      if (llvm::any_of(solution->Fixes, [](const auto *fix) {
+            return fix->getKind() == FixKind::AddSendableAttribute;
+          }))
+        return false;
+    }
 
     return true;
   }

test/Concurrency/sendable_keypaths.swift

@@ -217,3 +217,16 @@ do {
   let _: [PartialKeyPath<S>] = [\.a, \.b] // Ok
   let _: [any PartialKeyPath<S> & Sendable] = [\.a, \.b] // Ok
 }
+
+do {
+  func kp() -> KeyPath<String, Int> & Sendable {
+    fatalError()
+  }
+
+  func test() -> KeyPath<String, Int> {
+    true ? kp() : kp() // Ok
+  }
+
+  func forward<T>(_ v: T) -> T { v }
+  let _: KeyPath<String, Int> = forward(kp()) // Ok
+}

test/Concurrency/sendable_methods.swift

@@ -228,3 +228,19 @@ do {
     }
   }
 }
+
+do {
+  struct Test {
+    static func fn() {}
+    static func otherFn() {}
+  }
+
+  func fnRet(cond: Bool) -> () -> Void {
+    cond ? Test.fn : Test.otherFn // Ok
+  }
+
+  func forward<T>(_: T) -> T {
+  }
+
+  let _: () -> Void = forward(Test.fn) // Ok
+}