[Clang importer] Existentials do not conform to Hashable.

Fixes rdar://problem/47564982.

(cherry picked from commit 9084682)

Author: DougGregor

lib/ClangImporter/ImportType.cpp

@@ -23,6 +23,7 @@
 #include "swift/AST/Decl.h"
 #include "swift/AST/DiagnosticEngine.h"
 #include "swift/AST/DiagnosticsClangImporter.h"
+#include "swift/AST/ExistentialLayout.h"
 #include "swift/AST/GenericEnvironment.h"
 #include "swift/AST/Module.h"
 #include "swift/AST/NameLookup.h"
@@ -2441,7 +2442,11 @@ bool ClangImporter::Implementation::matchesHashableBound(Type type) {
     }
   }
 
-  // Class type or existential that inherits from NSObject.
+  // Existentials cannot match the Hashable bound.
+  if (type->isAnyExistentialType())
+    return false;
+
+  // Class type that inherits from NSObject.
   if (NSObjectType->isExactSuperclassOf(type))
     return true;
 

test/ClangImporter/Inputs/custom-modules/ObjCBridgeNonconforming.h

@@ -27,3 +27,7 @@
 @interface ObjCBridgeGenericConstrainedExtra<Element: NSObject <ExtraElementProtocol> *> : NSObject
 @property NSSet<Element> * _Nonnull foo;
 @end
+
+@interface ObjCBridgeExistential : NSObject
+@property NSSet<NSObject<ExtraElementProtocol> *> * _Nonnull foo;
+@end

test/ClangImporter/objc_bridging_generics.swift

@@ -408,11 +408,13 @@ func testHashableGenerics(
     any: ObjCBridgeGeneric<ElementConcrete>,
     constrained: ObjCBridgeGenericConstrained<ElementConcrete>,
     insufficient: ObjCBridgeGenericInsufficientlyConstrained<ElementConcrete>,
-    extra: ObjCBridgeGenericConstrainedExtra<ElementConcrete>) {
+    extra: ObjCBridgeGenericConstrainedExtra<ElementConcrete>,
+    existential: ObjCBridgeExistential) {
   let _: Int = any.foo // expected-error{{cannot convert value of type 'Set<AnyHashable>' to specified type 'Int'}}
   let _: Int = constrained.foo // expected-error{{cannot convert value of type 'Set<ElementConcrete>' to specified type 'Int'}}
   let _: Int = insufficient.foo // expected-error{{cannot convert value of type 'Set<AnyHashable>' to specified type 'Int'}}
   let _: Int = extra.foo // expected-error{{cannot convert value of type 'Set<ElementConcrete>' to specified type 'Int'}}
+  let _: Int = existential.foo // expected-error{{cannot convert value of type 'Set<AnyHashable>' to specified type 'Int'}}
 }
 
 func testGenericsWithTypedefBlocks(hba: HasBlockArray) {