[Typechecker] Relax existential to concrete type cast check for non-final classes

lib/Sema/TypeCheckConstraints.cpp

@@ -4298,9 +4298,13 @@ CheckedCastKind TypeChecker::typeCheckCheckedCast(Type fromType,
     //     print("Caught bar error")
     //   }
     // }
-    if (fromExistential) {
+    //
+    // Note: we relax the restriction if the type we're casting to is a
+    // non-final class because it's possible that we might have a subclass
+    // that conforms to the protocol.
+    if (fromExistential && !toExistential) {
       if (auto NTD = toType->getAnyNominal()) {
-        if (!isa<ProtocolDecl>(NTD)) {
+        if (!toType->is<ClassType>() || NTD->isFinal()) {
           auto protocolDecl =
               dyn_cast_or_null<ProtocolDecl>(fromType->getAnyNominal());
           if (protocolDecl &&

test/Constraints/patterns.swift

@@ -55,7 +55,7 @@ case is B,
      is D,
      is S:
   ()
-case is E: // expected-warning {{cast from 'P' to unrelated type 'E' always fails}}
+case is E:
   ()
 default:
   ()
@@ -69,7 +69,7 @@ case let d as D:
   d.d()
 case let s as S:
   s.s()
-case let e as E: // expected-warning {{cast from 'P' to unrelated type 'E' always fails}}
+case let e as E:
   e.e()
 default:
   ()

test/stmt/errors.swift

@@ -209,17 +209,37 @@ class SR_6400_B: SR_6400_FakeApplicationDelegate & Error {}
 func sr_6400_4() {
   do {
     throw SR_6400_E.castError
-  } catch let error as SR_6400_A { // expected-warning {{cast from 'Error' to unrelated type 'SR_6400_A' always fails}} // expected-warning {{immutable value 'error' was never used; consider replacing with '_' or removing it}}
-    print("Foo")
+  } catch let error as SR_6400_A { // Okay
+    print(error)
   } catch {
     print("Bar")
   }
 
   do {
     throw SR_6400_E.castError
-  } catch let error as SR_6400_B { // expected-warning {{immutable value 'error' was never used; consider replacing with '_' or removing it}}
-    print("Foo")
+  } catch let error as SR_6400_B { // Okay
+    print(error)
   } catch {
     print("Bar")
   }
 }
+
+// SR-11402
+
+protocol SR_11402_P {}
+class SR_11402_Superclass {}
+class SR_11402_Subclass: SR_11402_Superclass, SR_11402_P {}
+
+func sr_11402_func1(_ x: SR_11402_P) {
+  if let y = x as? SR_11402_Superclass { // Okay
+    print(y)
+  }
+}
+
+final class SR_11402_Final {}
+
+func sr_11402_func2(_ x: SR_11402_P) {
+  if let y = x as? SR_11402_Final { // expected-warning {{cast from 'SR_11402_P' to unrelated type 'SR_11402_Final' always fails}}
+    print(y)
+  }
+}