[typechecker] remove call to getAllConformances() and add some new test cases

Author: Suyash Srijan

lib/Sema/TypeCheckConstraints.cpp

@@ -4040,22 +4040,21 @@ CheckedCastKind TypeChecker::typeCheckCheckedCast(Type fromType,
     if (toType->isAnyObject() || fromType->isAnyObject())
       return CheckedCastKind::ValueCast;
 
-    // A cast from an existential type to a concrete type does not succeed.
+    // A cast from an existential type to a concrete type does not succeed. For
+    // example:
     //
     // struct S {}
     // enum FooError: Error { case bar }
-    // func foo() throws {
+    //
+    // func foo() {
     //   do {
     //     throw FooError.bar
-    //   } catch is X {
+    //   } catch is X { /* Will always fail */
     //     print("Caught bar error")
     //   }
     // }
     if (fromExistential) {
       if (auto NTD = toType->getAnyNominal()) {
-        if (NTD->getAllConformances().empty()) {
-          return failed();
-        }
 
         auto protocolDecl =
             dyn_cast_or_null<ProtocolDecl>(fromType->getAnyNominal());

test/stmt/errors.swift

@@ -175,16 +175,51 @@ enum SR_6400_E: Error {
 struct SR_6400_S_1 {}
 struct SR_6400_S_2: Error {}
 
+protocol SR_6400_FakeApplicationDelegate: AnyObject {}
+class SR_6400_FakeViewController {}
+
 func sr_6400() throws {
   do {
     throw SR_6400_E.castError
   } catch is SR_6400_S_1 { // expected-warning {{cast from 'Error' to unrelated type 'SR_6400_S_1' always fails}}
     print("Caught error")
   }
-
+  
   do {
     throw SR_6400_E.castError
   } catch is SR_6400_S_2 {
     print("Caught error") // Ok
   }
 }
+
+func sr_6400_1<T>(error: Error, as type: T.Type) -> Bool {
+  return (error as? T) != nil // Ok
+}
+
+func sr_6400_2(error: Error) {
+  _ = error as? (SR_6400_FakeViewController & Error) // Ok
+}
+func sr_6400_3(error: Error) {
+  _ = error as? (Error & SR_6400_FakeApplicationDelegate) // Ok
+}
+
+class SR_6400_A {}
+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 {
+    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 {
+    print("Bar")
+  }
+}