[Frontend] Support underlying module for an aliased module

Author: xymus

lib/Sema/ImportResolution.cpp

@@ -371,7 +371,8 @@ getModuleImpl(ImportPath::Module modulePath, ModuleDecl *loadingModule,
   //
   // FIXME: We'd like to only use this in SIL mode, but unfortunately we use it
   // for clang overlays as well.
-  if (moduleID.Item == loadingModule->getName() && modulePath.size() == 1) {
+  if (ctx.getRealModuleName(moduleID.Item) == loadingModule->getName() &&
+      modulePath.size() == 1) {
     if (auto importer = ctx.getClangModuleLoader())
       return importer->loadModule(moduleID.Loc, modulePath);
     return nullptr;

test/Sema/module-alias-with-underlying-module.swift

@@ -0,0 +1,31 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: %target-swift-frontend -typecheck -module-name MyModule \
+// RUN:     -swift-version 5 \
+// RUN:     -o %t/MyModule.swiftmodule \
+// RUN:     %t/MyModuleOk.swift -I%t
+
+// RUN: %target-swift-frontend -typecheck -module-name MyModule \
+// RUN:     -swift-version 5 \
+// RUN:     -o %t/MyModule.swiftmodule \
+// RUN:     %t/MyModule.swift -I%t \
+// RUN:     -module-alias Mod___MyModule=MyModule
+
+//--- module.modulemap
+module MyModule {
+    header "MyModule.h"
+}
+
+//--- MyModule.h
+struct UnderlyingType {};
+
+//--- MyModuleOk.swift
+
+@_exported import MyModule
+public func refToUnderlying(_ a: MyModule.UnderlyingType) {}
+
+//--- MyModule.swift
+
+@_exported import Mod___MyModule
+public func refToUnderlying(_ a: Mod___MyModule.UnderlyingType) {}