[typechecker] if we're casting to a nominal type that's a protocol, then skip the check

Author: Suyash Srijan

lib/Sema/TypeCheckConstraints.cpp

@@ -4055,13 +4055,14 @@ CheckedCastKind TypeChecker::typeCheckCheckedCast(Type fromType,
     // }
     if (fromExistential) {
       if (auto NTD = toType->getAnyNominal()) {
-
-        auto protocolDecl =
-            dyn_cast_or_null<ProtocolDecl>(fromType->getAnyNominal());
-        if (protocolDecl &&
-            !conformsToProtocol(toType, protocolDecl, dc,
-                                ConformanceCheckFlags::InExpression)) {
-          return failed();
+        if (!isa<ProtocolDecl>(NTD)) {
+          auto protocolDecl =
+              dyn_cast_or_null<ProtocolDecl>(fromType->getAnyNominal());
+          if (protocolDecl &&
+              !conformsToProtocol(toType, protocolDecl, dc,
+                                  ConformanceCheckFlags::InExpression)) {
+            return failed();
+          }
         }
       }
     }

test/Constraints/casts.swift

@@ -153,9 +153,9 @@ func objc_protocol_casts(_ op1: ObjCProto1, opn: NonObjCProto) {
   _ = ObjCClass() as! ObjCProto1 & NonObjCProto
 
   _ = op1 as! ObjCProto1 & ObjCProto2
-  _ = op1 as! ObjCProto2 // expected-warning {{cast from 'ObjCProto1' to unrelated type 'ObjCProto2' always fails}}
+  _ = op1 as! ObjCProto2
   _ = op1 as! ObjCProto1 & NonObjCProto
-  _ = opn as! ObjCProto1 // expected-warning {{cast from 'NonObjCProto' to unrelated type 'ObjCProto1' always fails}}
+  _ = opn as! ObjCProto1
 
   _ = NonObjCClass() as! ObjCProto1
 }