[Mangler] Mangle compatibility aliases as decls synthesized by ClangImporter

Author: a7medev

lib/AST/ASTMangler.cpp

@@ -2523,6 +2523,14 @@ ASTMangler::getSpecialManglingContext(const ValueDecl *decl,
   if (auto file = dyn_cast<FileUnit>(decl->getDeclContext())) {
     if (file->getKind() == FileUnitKind::ClangModule ||
         file->getKind() == FileUnitKind::DWARFModule) {
+
+      // Use ClangImporterContext for compatibility aliases to avoid conflicting
+      // with the mangled name of the actual declaration.
+      if (auto *aliasDecl = dyn_cast<TypeAliasDecl>(decl)) {
+        if (aliasDecl->isCompatibilityAlias())
+          return ASTMangler::ClangImporterContext;
+      }
+
       if (decl->getClangDecl())
         return ASTMangler::ObjCContext;
       return ASTMangler::ClangImporterContext;
@@ -3116,6 +3124,13 @@ void ASTMangler::appendAnyGenericType(const GenericTypeDecl *decl,
   // Always use Clang names for imported Clang declarations, unless they don't
   // have one.
   auto tryAppendClangName = [this, decl]() -> bool {
+    // We use the Swift name rather than the Clang name for compatibility
+    // aliases to avoid multiple declarations having the same mangled name.
+    if (auto *aliasDecl = dyn_cast<TypeAliasDecl>(decl)) {
+      if (aliasDecl->isCompatibilityAlias())
+        return false;
+    }
+
     auto *nominal = dyn_cast<NominalTypeDecl>(decl);
     auto namedDecl = getClangDeclForMangling(decl);
     if (!namedDecl) {

lib/ClangImporter/ClangImporter.cpp

@@ -3551,6 +3551,17 @@ void ClangImporter::lookupTypeDecl(
         if (auto ext = dyn_cast_or_null<ExtensionDecl>(imported)) {
           imported = ext->getExtendedNominal();
         }
+
+        // Look through compatibility aliases since we must have mangled the
+        // underlying type (see ASTMangler::getSpecialManglingContext).
+        if (auto *alias = dyn_cast_or_null<TypeAliasDecl>(imported)) {
+          if (alias->isCompatibilityAlias()) {
+            imported = alias->getUnderlyingType()->getAnyNominal();
+            assert(imported != nullptr &&
+                   "No underlying decl for a compatibility typealias");
+          }
+        }
+
         if (auto *importedType = dyn_cast_or_null<TypeDecl>(imported)) {
           foundViaClang = true;
           receiver(importedType);

test/APINotes/versioned-test-mangling.swift

@@ -8,7 +8,7 @@
 
 import APINotesFrameworkTest
 
-// CHECK-TOP-ALIAS-4: typealias ImportantCStruct = VeryImportantCStruct
+// CHECK-TOP-ALIAS-4: struct VeryImportantCStruct {
 // CHECK-TOP-ALIAS-5: struct VeryImportantCStruct {
-// CHECK-NESTED-ALIAS-4: typealias InnerInSwift5 = Outer.Inner
+// CHECK-NESTED-ALIAS-4: struct Inner {
 // CHECK-NESTED-ALIAS-5: struct Inner {