[Clang importer] Rename ImportTypeKind::ObjCGenericArgument -> ObjCCollectionElement.

This better describes what we're importing. Also note why we need to
reset to the block representation.

Author: DougGregor

lib/ClangImporter/ImportType.cpp

@@ -974,7 +974,7 @@ namespace {
           // Convert the type arguments.
           for (auto typeArg : typeArgs) {
             Type importedTypeArg = Impl.importTypeIgnoreIUO(
-                typeArg, ImportTypeKind::ObjCGenericArgument,
+                typeArg, ImportTypeKind::ObjCCollectionElement,
                 AllowNSUIntegerAsInt, Bridging, OTK_None);
             if (!importedTypeArg) {
               importedTypeArgs.clear();
@@ -1102,7 +1102,7 @@ static bool canBridgeTypes(ImportTypeKind importKind) {
   case ImportTypeKind::CFUnretainedOutParameter:
   case ImportTypeKind::Property:
   case ImportTypeKind::PropertyWithReferenceSemantics:
-  case ImportTypeKind::ObjCGenericArgument:
+  case ImportTypeKind::ObjCCollectionElement:
   case ImportTypeKind::Typedef:
     return true;
   }
@@ -1116,7 +1116,7 @@ static bool isCFAudited(ImportTypeKind importKind) {
   case ImportTypeKind::Abstract:
   case ImportTypeKind::Typedef:
   case ImportTypeKind::Value:
-  case ImportTypeKind::ObjCGenericArgument:
+  case ImportTypeKind::ObjCCollectionElement:
   case ImportTypeKind::Variable:
   case ImportTypeKind::Result:
   case ImportTypeKind::Pointee:
@@ -1287,12 +1287,17 @@ static ImportedType adjustTypeForConcreteImport(
   // we would prefer to instead use the default Swift convention.
   if (hint == ImportHint::Block) {
     if (canBridgeTypes(importKind)) {
-      // Determine the representation we need. For Objective-C generic
-      // arguments, we cannot bridge them to a block, so force a block
-      // representation even if our imported type thus far is a Swift
-      // function representation.
+      // Determine the function type representation we need.
+      //
+      // For Objective-C collection arguments, we cannot bridge from a block
+      // to a Swift function type, so force the block representation. Normally
+      // the mapped type will have a block representation (making this a no-op),
+      // but in cases where the Clang type was written as a typedef of a
+      // block type, that typedef will have a Swift function type
+      // representation. This code will then break down the imported type
+      // alias and produce a function type with block representation.
       auto requiredFunctionTypeRepr = FunctionTypeRepresentation::Swift;
-      if (importKind == ImportTypeKind::ObjCGenericArgument) {
+      if (importKind == ImportTypeKind::ObjCCollectionElement) {
         requiredFunctionTypeRepr = FunctionTypeRepresentation::Block;
       }
 

lib/ClangImporter/ImporterImpl.h

@@ -109,7 +109,7 @@ enum class ImportTypeKind {
   Value,
 
   /// \brief Import the type of an Objective-C generic argument.
-  ObjCGenericArgument,
+  ObjCCollectionElement,
 
   /// \brief Import the declared type of a variable.
   Variable,