Merge pull request swiftlang#23217 from theblixguy/fix/SR-6400

[Typechecker] Warn when casting from an existential type to a concrete type

Author: xedin

lib/Sema/TypeCheckConstraints.cpp

@@ -4040,6 +4040,33 @@ 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. For
+    // example:
+    //
+    // struct S {}
+    // enum FooError: Error { case bar }
+    //
+    // func foo() {
+    //   do {
+    //     throw FooError.bar
+    //   } catch is X { /* Will always fail */
+    //     print("Caught bar error")
+    //   }
+    // }
+    if (fromExistential) {
+      if (auto NTD = toType->getAnyNominal()) {
+        if (!isa<ProtocolDecl>(NTD)) {
+          auto protocolDecl =
+              dyn_cast_or_null<ProtocolDecl>(fromType->getAnyNominal());
+          if (protocolDecl &&
+              !conformsToProtocol(toType, protocolDecl, dc,
+                                  ConformanceCheckFlags::InExpression)) {
+            return failed();
+          }
+        }
+      }
+    }
+
     bool toRequiresClass;
     if (toType->isExistentialType())
       toRequiresClass = toType->getExistentialLayout().requiresClass();

test/Constraints/casts.swift

@@ -111,7 +111,7 @@ func protocol_concrete_casts(_ p1: P1, p2: P2, p12: P1 & P2) {
 
   _ = p1 as! P1 & P2
 
-  _ = p2 as! S1
+  _ = p2 as! S1 // expected-warning {{cast from 'P2' to unrelated type 'S1' always fails}}
 
   _ = p12 as! S1
   _ = p12 as! S2
@@ -126,7 +126,7 @@ func protocol_concrete_casts(_ p1: P1, p2: P2, p12: P1 & P2) {
 
   var _:Bool = p1 is P1 & P2
 
-  var _:Bool = p2 is S1
+  var _:Bool = p2 is S1 // expected-warning {{cast from 'P2' to unrelated type 'S1' always fails}}
 
   var _:Bool = p12 is S1
   var _:Bool = p12 is S2

test/Constraints/patterns.swift

@@ -55,7 +55,7 @@ case is B,
      is D,
      is S:
   ()
-case is E:
+case is E: // expected-warning {{cast from 'P' to unrelated type 'E' always fails}}
   ()
 default:
   ()
@@ -69,7 +69,7 @@ case let d as D:
   d.d()
 case let s as S:
   s.s()
-case let e as E:
+case let e as E: // expected-warning {{cast from 'P' to unrelated type 'E' always fails}}
   e.e()
 default:
   ()
@@ -120,7 +120,7 @@ case iPadHair<E>.HairForceOne:
   ()
 case iPadHair.HairForceOne: // expected-error{{generic enum type 'iPadHair' is ambiguous without explicit generic parameters when matching value of type 'HairType'}}
   ()
-case Watch.Edition: // TODO: should warn that cast can't succeed with currently known conformances
+case Watch.Edition: // expected-warning {{cast from 'HairType' to unrelated type 'Watch' always fails}}
   ()
 case .HairForceOne: // expected-error{{type 'HairType' has no member 'HairForceOne'}}
   ()

test/stmt/errors.swift

@@ -165,3 +165,61 @@ func thirteen() {
     }
   }
 }
+
+// SR 6400
+
+enum SR_6400_E: Error {
+  case castError
+}
+
+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")
+  }
+}