diff --git a/include/swift/Basic/FileTypes.def b/include/swift/Basic/FileTypes.def
index f2a832821a0..5ac6275ea52 100644
--- a/include/swift/Basic/FileTypes.def
+++ b/include/swift/Basic/FileTypes.def
@@ -73,6 +73,13 @@ TYPE("module-trace",        ModuleTrace,               "trace.json",      "")
 TYPE("index-data",          IndexData,                 "",                "")
 TYPE("opt-record",          OptRecord,                 "opt.yaml",        "")
 
+// Overlay files declare wrapper modules, called "separately-imported overlays",
+// that should be automatically imported when a particular module is imported.
+// Cross-import directories conditionalize overlay files so they only take
+// effect when certain other modules are also loaded.
+TYPE("swiftcrossimport",    SwiftCrossImportDir,       "swiftcrossimport","")
+TYPE("swiftoverlay",        SwiftOverlayFile,          "swiftoverlay",    "")
+
 // Misc types
 TYPE("pcm",                 ClangModuleFile,           "pcm",             "")
 TYPE("pch",                 PCH,                       "pch",             "")
diff --git a/lib/Basic/FileTypes.cpp b/lib/Basic/FileTypes.cpp
index bae29a9f0da..66222299748 100644
--- a/lib/Basic/FileTypes.cpp
+++ b/lib/Basic/FileTypes.cpp
@@ -80,6 +80,7 @@ bool file_types::isTextual(ID Id) {
   case file_types::TY_ModuleTrace:
   case file_types::TY_OptRecord:
   case file_types::TY_SwiftModuleInterfaceFile:
+  case file_types::TY_SwiftOverlayFile:
     return true;
   case file_types::TY_Image:
   case file_types::TY_Object:
@@ -90,6 +91,7 @@ bool file_types::isTextual(ID Id) {
   case file_types::TY_SwiftModuleFile:
   case file_types::TY_SwiftModuleDocFile:
   case file_types::TY_SwiftSourceInfoFile:
+  case file_types::TY_SwiftCrossImportDir:
   case file_types::TY_LLVM_BC:
   case file_types::TY_SerializedDiagnostics:
   case file_types::TY_ClangModuleFile:
@@ -132,6 +134,8 @@ bool file_types::isAfterLLVM(ID Id) {
   case file_types::TY_SwiftModuleFile:
   case file_types::TY_SwiftModuleDocFile:
   case file_types::TY_SwiftSourceInfoFile:
+  case file_types::TY_SwiftCrossImportDir:
+  case file_types::TY_SwiftOverlayFile:
   case file_types::TY_SerializedDiagnostics:
   case file_types::TY_ClangModuleFile:
   case file_types::TY_SwiftDeps:
@@ -176,6 +180,8 @@ bool file_types::isPartOfSwiftCompilation(ID Id) {
   case file_types::TY_SwiftModuleDocFile:
   case file_types::TY_SwiftModuleInterfaceFile:
   case file_types::TY_SwiftSourceInfoFile:
+  case file_types::TY_SwiftCrossImportDir:
+  case file_types::TY_SwiftOverlayFile:
   case file_types::TY_SerializedDiagnostics:
   case file_types::TY_ClangModuleFile:
   case file_types::TY_SwiftDeps:
diff --git a/lib/Driver/Driver.cpp b/lib/Driver/Driver.cpp
index 2eb888a76be..99dda146e75 100644
--- a/lib/Driver/Driver.cpp
+++ b/lib/Driver/Driver.cpp
@@ -1942,6 +1942,8 @@ void Driver::buildActions(SmallVectorImpl<const Action *> &TopLevelActions,
       case file_types::TY_ModuleTrace:
       case file_types::TY_OptRecord:
       case file_types::TY_SwiftModuleInterfaceFile:
+      case file_types::TY_SwiftCrossImportDir:
+      case file_types::TY_SwiftOverlayFile:
         // We could in theory handle assembly or LLVM input, but let's not.
         // FIXME: What about LTO?
         Diags.diagnose(SourceLoc(), diag::error_unexpected_input_file,
diff --git a/lib/Driver/ToolChains.cpp b/lib/Driver/ToolChains.cpp
index cf9f91bea10..4d72023af6d 100644
--- a/lib/Driver/ToolChains.cpp
+++ b/lib/Driver/ToolChains.cpp
@@ -562,6 +562,8 @@ const char *ToolChain::JobContext::computeFrontendModeForCompile() const {
   case file_types::TY_OptRecord:
   case file_types::TY_SwiftModuleInterfaceFile:
   case file_types::TY_SwiftSourceInfoFile:
+  case file_types::TY_SwiftCrossImportDir:
+  case file_types::TY_SwiftOverlayFile:
     llvm_unreachable("Output type can never be primary output.");
   case file_types::TY_INVALID:
     llvm_unreachable("Invalid type ID");
@@ -811,6 +813,8 @@ ToolChain::constructInvocation(const BackendJobAction &job,
     case file_types::TY_OptRecord:
     case file_types::TY_SwiftModuleInterfaceFile:
     case file_types::TY_SwiftSourceInfoFile:
+    case file_types::TY_SwiftCrossImportDir:
+    case file_types::TY_SwiftOverlayFile:
       llvm_unreachable("Output type can never be primary output.");
     case file_types::TY_INVALID:
       llvm_unreachable("Invalid type ID");