Fix canImport submodule checking in loaders not supporting submodules

lib/Frontend/ModuleInterfaceLoader.cpp

@@ -1849,6 +1849,8 @@ bool ExplicitSwiftModuleLoader::canImportModule(ImportPath::Module path,
                                                 llvm::VersionTuple version,
                                                 bool underlyingVersion) {
   // FIXME: Swift submodules?
+  if (path.hasSubmodule())
+    return false;
   ImportPath::Element mID = path.front();
   // Look up the module with the real name (physical name on disk);
   // in case `-module-alias` is used, the name appearing in source files

lib/Sema/SourceLoader.cpp

@@ -73,7 +73,7 @@ bool SourceLoader::canImportModule(ImportPath::Module path,
                                    llvm::VersionTuple version,
                                    bool underlyingVersion) {
   // FIXME: Swift submodules?
-  if (path.size() > 1)
+  if (path.hasSubmodule())
     return false;
 
   auto ID = path[0];

lib/Serialization/SerializedModuleLoader.cpp

@@ -1133,6 +1133,9 @@ swift::extractUserModuleVersionFromInterface(StringRef moduleInterfacePath) {
 bool SerializedModuleLoaderBase::canImportModule(ImportPath::Module path,
                                                  llvm::VersionTuple version,
                                                  bool underlyingVersion) {
+  // FIXME: Swift submodules?
+  if (path.hasSubmodule())
+    return false;
   // If underlying version is specified, this should be handled by Clang importer.
   if (!version.empty() && underlyingVersion)
     return false;
@@ -1153,7 +1156,6 @@ bool SerializedModuleLoaderBase::canImportModule(ImportPath::Module path,
     unusedModuleDocBuffer = &moduleDocBuffer;
   }
 
-  // FIXME: Swift submodules?
   auto mID = path[0];
   auto found = findModule(mID, unusedModuleInterfacePath, unusedModuleBuffer,
                           unusedModuleDocBuffer, unusedModuleSourceInfoBuffer,
@@ -1192,10 +1194,12 @@ bool SerializedModuleLoaderBase::canImportModule(ImportPath::Module path,
 bool MemoryBufferSerializedModuleLoader::canImportModule(
     ImportPath::Module path, llvm::VersionTuple version,
     bool underlyingVersion) {
+  // FIXME: Swift submodules?
+  if (path.hasSubmodule())
+    return false;
   // If underlying version is specified, this should be handled by Clang importer.
   if (!version.empty() && underlyingVersion)
     return false;
-  // FIXME: Swift submodules?
   auto mID = path[0];
   auto mIt = MemoryBuffers.find(mID.Item.str());
   if (mIt == MemoryBuffers.end())

test/ClangImporter/MixedSource/Inputs/WithSubmodule.framework/Headers/Submodule.h

@@ -0,0 +1 @@
+int fromSubmodule;

test/ClangImporter/MixedSource/Inputs/WithSubmodule.framework/module.map

@@ -0,0 +1,6 @@
+framework module WithSubmodule {
+  explicit module Submodule {
+    header "Submodule.h"
+    export *
+  }
+}

test/ClangImporter/MixedSource/can_import_submodule_in_mixed_framework.swift

@@ -0,0 +1,21 @@
+// RUN: %empty-directory(%t)
+// RUN: cp -r %S/Inputs/WithSubmodule.framework %t
+// RUN: %target-swift-frontend -emit-module -o %t/WithSubmodule.framework/Modules/WithSubmodule.swiftmodule/%target-swiftmodule-name %t/WithSubmodule.framework/Empty.swift -import-underlying-module -F %t -module-name WithSubmodule
+
+// RUN: %target-typecheck-verify-swift -F %t
+
+// Testing 'canImport()' non-existing submodule in a top module loadable by other loaders.
+
+#if !canImport(WithSubmodule.Submodule)
+#error("Should can import WithSubmodule.Submodule")
+#endif
+
+// Should fail if checked for a non-existing submodule.
+#if canImport(WithSubmodule.ButNotMe)
+import WithSubmodule.Submodule
+#endif
+
+func testNotImported() {
+    fromSubmodule = 5
+    // expected-error@-1 {{cannot find 'fromSubmodule' in scope}}
+}