[Runtime] Handle existential target types in swift_dynamicCastMetatypeImpl.

This fixes cases like `type is T.Type` when T is generic specialized to a protocol type.

Note: the compiler can still optimize these checks away and will optimize this check down to `false` in some cases. We'll want to fix that as well.

rdar://problem/56044443

Author: mikeash

stdlib/public/runtime/Casting.cpp

@@ -1115,6 +1115,14 @@ swift_dynamicCastMetatypeImpl(const Metadata *sourceType,
     }
     break;
 
+  case MetadataKind::Existential: {
+    auto targetTypeAsExistential = static_cast<const ExistentialTypeMetadata *>(targetType);
+    for (auto protocol : targetTypeAsExistential->getProtocols())
+      if (!swift_conformsToProtocol(sourceType, protocol.getSwiftProtocol()))
+        return nullptr;
+    return origSourceType;
+  }
+
   default:
     // The cast succeeds only if the metadata pointers are statically
     // equivalent.

test/Interpreter/generic_casts.swift

@@ -1,4 +1,4 @@
-// RUN: %target-run-simple-swift | %FileCheck %s
+// RUN: %target-run-simple-swift | %FileCheck --check-prefix CHECK --check-prefix CHECK-ONONE %s
 // RUN: %target-build-swift -O %s -o %t/a.out.optimized
 // RUN: %target-codesign %t/a.out.optimized
 // RUN: %target-run %t/a.out.optimized | %FileCheck %s
@@ -101,6 +101,24 @@ func allMetasToAllMetas<T, U>(_: T.Type, _: U.Type) -> Bool {
   return T.self is U.Type
 }
 
+protocol P {}
+struct PS: P {}
+enum PE: P {}
+class PC: P {}
+
+func nongenericAnyIsP(type: Any.Type) -> Bool {
+  return type is P.Type
+}
+func genericAnyIs<T>(type: Any.Type, to: T.Type) -> Bool {
+  return type is T.Type
+}
+print("nongenericAnyIsP(type: PS.self)", nongenericAnyIsP(type: PS.self)) // CHECK: nongenericAnyIsP(type: PS.self) true
+print("genericAnyIs(type: PS.self, to: P.self)", genericAnyIs(type: PS.self, to: P.self)) // CHECK-ONONE: genericAnyIs(type: PS.self, to: P.self) true
+print("nongenericAnyIsP(type: PE.self)", nongenericAnyIsP(type: PE.self)) // CHECK: nongenericAnyIsP(type: PE.self) true
+print("genericAnyIs(type: PE.self, to: P.self)", genericAnyIs(type: PE.self, to: P.self)) // CHECK-ONONE: genericAnyIs(type: PE.self, to: P.self) true
+print("nongenericAnyIsP(type: PC.self)", nongenericAnyIsP(type: PC.self)) // CHECK: nongenericAnyIsP(type: PC.self) true
+print("genericAnyIs(type: PC.self, to: P.self)", genericAnyIs(type: PC.self, to: P.self)) // CHECK-ONONE: genericAnyIs(type: PC.self, to: P.self) true
+
 print(allToInt(22)) // CHECK: 22
 print(anyToInt(44)) // CHECK: 44
 allToC(C()).print() // CHECK: C!