[Clang importer] Type parameters substituted with existentials aren't Hashable

As a follow-on to the fix for rdar://problem/47564982, a type
parameter in a generic Objective-C class whose type bounds include
both a NSObject-derived superclass and a protocol would have been
classified has being Hashable, when in fact the corresponding
existential type will not conform to Hashable. Only allow hashability
when there are no protocol requirements.

Author: DougGregor

lib/ClangImporter/ImportType.cpp

@@ -2436,9 +2436,13 @@ bool ClangImporter::Implementation::matchesHashableBound(Type type) {
   // Match generic parameters against their bounds.
   if (auto *genericTy = type->getAs<GenericTypeParamType>()) {
     if (auto *generic = genericTy->getDecl()) {
-      type = generic->getSuperclass();
-      if (!type)
-        return false;
+      auto genericSig =
+        generic->getDeclContext()->getGenericSignatureOfContext();
+      if (genericSig && genericSig->getConformsTo(type).empty()) {
+        type = genericSig->getSuperclassBound(type);
+        if (!type)
+          return false;
+      }
     }
   }
 

test/ClangImporter/objc_bridging_generics.swift

@@ -413,7 +413,7 @@ func testHashableGenerics(
   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 = extra.foo // expected-error{{cannot convert value of type 'Set<AnyHashable>' to specified type 'Int'}}
   let _: Int = existential.foo // expected-error{{cannot convert value of type 'Set<AnyHashable>' to specified type 'Int'}}
 }