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

Author: theblixguy

lib/Sema/TypeCheckConstraints.cpp

@@ -4298,7 +4298,11 @@ 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 class
+    // because it's possible that we might have a subclass that conforms to
+    // the protocol.
+    if (fromExistential && !toType->hasReferenceSemantics()) {
       if (auto NTD = toType->getAnyNominal()) {
         if (!isa<ProtocolDecl>(NTD)) {
           auto protocolDecl =

test/stmt/errors.swift

@@ -209,7 +209,7 @@ 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}}
+  } catch let error as SR_6400_A { // expected-warning {{immutable value 'error' was never used; consider replacing with '_' or removing it}}
     print("Foo")
   } catch {
     print("Bar")
@@ -223,3 +223,17 @@ func sr_6400_4() {
     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_func(_ x: SR_11402_P) {
+  if let y = x as? SR_11402_Superclass { // Okay
+    print(y)
+  }
+}
+
+sr_11402_func(SR_11402_Subclass())