Sema: function casts ignore marker protocols

Dynamically they have no influence on whether the cast will succeed or
not, as there is no way to tell if a type conforms to such a protocol at
runtime.

Author: kavon

lib/Sema/TypeCheckConstraints.cpp

@@ -1964,7 +1964,9 @@ TypeChecker::typeCheckCheckedCast(Type fromType, Type toType,
   auto toConstrainedArchetype = false;
   if (toArchetypeType) {
     auto archetype = toType->castTo<ArchetypeType>();
-    toConstrainedArchetype = !archetype->getConformsTo().empty();
+    // If the archetype conforms to any non-marker protocols, it's constrained.
+    toConstrainedArchetype = llvm::any_of(archetype->getConformsTo(),
+                [](ProtocolDecl *proto){ return !proto->isMarkerProtocol(); });
   }
 
   if (fromFunctionType &&

test/Interpreter/casts.swift

@@ -142,3 +142,13 @@ Casts.test("testCastProtocolWithAnyObjectToProtocolCompoTypeSuperclass") {
   let shouldBeNil = (F() as QAny) as? KQAny
   expectNil(shouldBeNil)
 }
+
+func f1() -> Any { return 1 }
+func attemptFunctionCast<U>(_ u: U.Type) -> U? {
+  return f1 as? U
+}
+Casts.test("testFunctionCastToArchetype") {
+  expectNil(attemptFunctionCast(Int.self))
+  expectNil(attemptFunctionCast(K.self))
+  expectTrue(attemptFunctionCast(Any.self) != nil)
+}

test/expr/cast/as_coerce.swift

@@ -133,20 +133,29 @@ do {
   _ = f2 as? Any // expected-warning {{conditional cast from '() -> ()' to 'Any' always succeeds}}
 
 
-  func test1<T: P_48579>(_: T.Type) {
+  func test1<T: P_48579, V: P_48579 & Sendable>(_: T.Type, _: V.Type) {
     _ = f1 is T // expected-warning {{cast from '() -> Any' to unrelated type 'T' always fails}} // expected-note {{did you mean to call 'f1' with '()'?}}{{11-11=()}}
-    _ = f1 as! T // expected-warning {{cast from '() -> Any' to unrelated type 'T' always fails}} // expected-note {{did you mean to call 'f1' with '()'?}}{{11-11=()}}
+    _ = f1 as! V // expected-warning {{cast from '() -> Any' to unrelated type 'V' always fails}} // expected-note {{did you mean to call 'f1' with '()'?}}{{11-11=()}}
     _ = f1 as? T // expected-warning {{cast from '() -> Any' to unrelated type 'T' always fails}} // expected-note {{did you mean to call 'f1' with '()'?}}{{11-11=()}}
     _ = f2 is T // expected-warning {{cast from '() -> ()' to unrelated type 'T' always fails}}
-    _ = f2 as! T // expected-warning {{cast from '() -> ()' to unrelated type 'T' always fails}}
+    _ = f2 as! V // expected-warning {{cast from '() -> ()' to unrelated type 'V' always fails}}
     _ = f2 as? T // expected-warning {{cast from '() -> ()' to unrelated type 'T' always fails}}
   }
 
-  func test2<U>(_: U.Type) {
+  func test2<U, S: Sendable>(_: U.Type, _: S.Type) {
+    _ = f1 is U  // Okay
     _ = f1 as! U // Okay
     _ = f1 as? U // Okay
+    _ = f1 is U  // Okay
     _ = f2 as! U // Okay
     _ = f2 as? U // Okay
+
+    _ = f1 is S  // Okay
+    _ = f1 as! S // Okay
+    _ = f1 as? S  // Okay
+    _ = f1 is S  // Okay
+    _ = f2 as! S // Okay
+    _ = f2 as? S // Okay
   }
 }