Dynamic casts to AnyObject should succeed for an arbitrary source

type because we can always make a SwiftValue.

rdar://26268575

Author: rjmccall

stdlib/public/runtime/Casting.cpp

@@ -1049,6 +1049,26 @@ static bool _dynamicCastToExistential(OpaqueValue *dest,
     MetadataKind kind =
         srcDynamicType ? srcDynamicType->getKind() : MetadataKind::Class;
 
+    // A fallback to use if we don't have a more specialized approach
+    // for a non-class type.
+    auto fallbackForNonClass = [&] {
+#if SWIFT_OBJC_INTEROP
+      // If the destination type is a set of protocols that SwiftValue
+      // implements, we're fine.
+      if (findSwiftValueConformances(targetType->Protocols,
+                                     destExistential->getWitnessTables())) {
+        bool consumeValue = dynamicFlags & DynamicCastFlags::TakeOnSuccess;
+        destExistential->Value =
+          bridgeAnythingToSwiftValueObject(srcDynamicValue, srcDynamicType,
+                                           consumeValue);
+        maybeDeallocateSource(true);
+        return true;
+      }
+#endif
+
+      return _fail(src, srcType, targetType, flags);
+    };
+
     // If the source type is a value type, it cannot possibly conform
     // to a class-bounded protocol. 
     switch (kind) {
@@ -1068,14 +1088,15 @@ static bool _dynamicCastToExistential(OpaqueValue *dest,
       }
 #endif
       // Otherwise, metatypes aren't class objects.
-      return _fail(src, srcType, targetType, flags);
+      return fallbackForNonClass();
     }
 
     case MetadataKind::Class:
     case MetadataKind::ObjCClassWrapper:
     case MetadataKind::ForeignClass:
     case MetadataKind::Existential:
-      // Handle these cases below.
+      // Handle the class cases below.  Note that opaque existentials
+      // shouldn't get here because we should have drilled into them above.
       break;
 
     case MetadataKind::Struct:
@@ -1105,16 +1126,15 @@ static bool _dynamicCastToExistential(OpaqueValue *dest,
         return success;
       }
 #endif
-      break;
+      SWIFT_FALLTHROUGH;
 
     case MetadataKind::Function:
     case MetadataKind::HeapLocalVariable:
     case MetadataKind::HeapGenericLocalVariable:
     case MetadataKind::ErrorObject:
     case MetadataKind::Opaque:
     case MetadataKind::Tuple:
-      // Will never succeed.
-      return _fail(src, srcType, targetType, flags);
+      return fallbackForNonClass();
     }
 
     // Check for protocol conformances and fill in the witness tables. If

stdlib/public/runtime/SwiftValue.h

@@ -23,6 +23,7 @@
 
 #if SWIFT_OBJC_INTEROP
 #include <objc/runtime.h>
+#include "swift/Runtime/Metadata.h"
 
 // _SwiftValue is an Objective-C class, but we shouldn't interface with it
 // directly as such. Keep the type opaque.
@@ -51,6 +52,12 @@ getValueFromSwiftValue(_SwiftValue *v);
 /// or nil if it is not.
 _SwiftValue *getAsSwiftValue(id object);
 
+/// Find conformances for SwiftValue to the given list of protocols.
+///
+/// Returns true if SwiftValue does conform to all the protocols.
+bool findSwiftValueConformances(const ProtocolDescriptorList &protocols,
+                                const WitnessTable **tablesBuffer);
+
 } // namespace swift
 #endif
 

stdlib/public/runtime/SwiftValue.mm

@@ -212,6 +212,49 @@ static size_t getSwiftValuePayloadAlignMask(const Metadata *type) {
   return nil;
 }
 
+bool
+swift::findSwiftValueConformances(const ProtocolDescriptorList &protocols,
+                                  const WitnessTable **tablesBuffer) {
+  Class cls = nullptr;
+
+  // Note that currently we never modify tablesBuffer because
+  // _SwiftValue doesn't conform to any protocols that need witness tables.
+
+  for (size_t i = 0, e = protocols.NumProtocols; i != e; ++i) {
+    auto protocol = protocols[i];
+
+    // _SwiftValue does conform to AnyObject.
+    switch (protocol->Flags.getSpecialProtocol()) {
+    case SpecialProtocol::AnyObject:
+      continue;
+
+    case SpecialProtocol::Error:
+      return false;
+
+    case SpecialProtocol::None:
+      break;
+    }
+
+    // Otherwise, it only conforms to ObjC protocols.  We specifically
+    // don't want to say that _SwiftValue conforms to the Swift protocols
+    // that NSObject conforms to because that would create a situation
+    // where arguably an arbitrary type would conform to those protocols
+    // by way of coercion through _SwiftValue.  Eventually we want to
+    // change _SwiftValue to not be an NSObject subclass at all.
+
+    if (protocol->Flags.getDispatchStrategy() != ProtocolDispatchStrategy::ObjC)
+      return false;
+
+    if (!cls) cls = _getSwiftValueClass();
+
+    // Check whether the class conforms to the protocol.
+    if (![cls conformsToProtocol: (Protocol*) protocol])
+      return false;
+  }
+
+  return true;
+}
+
 @implementation _SwiftValue
 
 + (instancetype)allocWithZone:(NSZone *)zone {

test/Interpreter/generic_casts.swift

@@ -158,8 +158,15 @@ print(allToAll(type(of: C()), AnyObject.self)) // CHECK-NEXT: true
 // Bridging
 print(allToAll(0, AnyObject.self)) // CHECK-NEXT: true
 
+// This will get bridged using _SwiftValue.
 struct NotBridged { var x: Int }
-print(allToAll(NotBridged(x: 0), AnyObject.self)) // CHECK-NEXT: false
+print(allToAll(NotBridged(x: 0), AnyObject.self)) // CHECK-NEXT: true
+print(allToAll(NotBridged(x: 0), NSCopying.self)) // CHECK-NEXT: true
+
+// These casts fail (intentionally) even though _SwiftValue does
+// technically conform to these protocols through NSObject.
+print(allToAll(NotBridged(x: 0), CustomStringConvertible.self)) // CHECK-NEXT: false
+print(allToAll(NotBridged(x: 0), (AnyObject & CustomStringConvertible).self)) // CHECK-NEXT: false
 
 //
 // rdar://problem/19482567