Import namespaces as EnumDecl. (#26339)

Author: pschuh

lib/AST/ASTMangler.cpp

@@ -1489,6 +1489,7 @@ ASTMangler::getSpecialManglingContext(const ValueDecl *decl,
         hasNameForLinkage = !clangDecl->getDeclName().isEmpty();
       if (hasNameForLinkage) {
         auto *clangDC = clangDecl->getDeclContext();
+        if (isa<clang::NamespaceDecl>(clangDC)) return None;
         assert(clangDC->getRedeclContext()->isTranslationUnit() &&
                "non-top-level Clang types not supported yet");
         (void)clangDC;
@@ -1831,6 +1832,10 @@ void ASTMangler::appendAnyGenericType(const GenericTypeDecl *decl) {
     } else if (isa<clang::TypedefNameDecl>(namedDecl) ||
                isa<clang::ObjCCompatibleAliasDecl>(namedDecl)) {
       appendOperator("a");
+    } else if (isa<clang::NamespaceDecl>(namedDecl)) {
+      // Note: Namespaces are not really structs, but since namespaces are
+      // imported as enums, be consistent.
+      appendOperator("V");
     } else {
       llvm_unreachable("unknown imported Clang type");
     }

lib/ClangImporter/ImportDecl.cpp

@@ -2373,8 +2373,25 @@ namespace {
     }
 
     Decl *VisitNamespaceDecl(const clang::NamespaceDecl *decl) {
-      // FIXME: Implement once Swift has namespaces.
-      return nullptr;
+      // If we have a name for this declaration, use it.
+      Optional<ImportedName> correctSwiftName;
+      auto importedName = importFullName(decl, correctSwiftName);
+      if (!importedName) return nullptr;
+
+      auto dc =
+          Impl.importDeclContextOf(decl, importedName.getEffectiveContext());
+      if (!dc)
+        return nullptr;
+
+      SourceLoc loc = Impl.importSourceLoc(decl->getBeginLoc());
+
+      // FIXME: If Swift gets namespaces, import as a namespace.
+      auto enumDecl = Impl.createDeclWithClangNode<EnumDecl>(
+          decl, AccessLevel::Public, loc,
+          importedName.getDeclName().getBaseIdentifier(),
+          Impl.importSourceLoc(decl->getLocation()), None, nullptr, dc);
+      enumDecl->computeType();
+      return enumDecl;
     }
 
     Decl *VisitUsingDirectiveDecl(const clang::UsingDirectiveDecl *decl) {

test/ClangImporter/Inputs/custom-modules/cxx_interop.h

@@ -11,3 +11,7 @@ struct Basic {
 };
 
 Basic makeA();
+
+ns::T* makeT();
+void useT(ns::T* v);
+using namespacedT = ns::T;

test/ClangImporter/cxx_interop_ir.swift

@@ -0,0 +1,14 @@
+// RUN: %target-swift-frontend -module-name cxx_ir -I %S/Inputs/custom-modules -module-cache-path %t -enable-cxx-interop -emit-ir -o - -primary-file %s | %FileCheck %s
+
+import CXXInterop
+
+// CHECK-LABEL: define hidden swiftcc void @"$s6cxx_ir13indirectUsageyyF"()
+// CHECK: %0 = call %"class.ns::T"* @_Z5makeTv()
+// CHECK: call void @_Z4useTPN2ns1TE(%"class.ns::T"* %2)
+func indirectUsage() {
+  useT(makeT())
+}
+
+// CHECK: define hidden swiftcc void @"$s6cxx_ir24namespaceManglesIntoName3argySo2nsV1TV_tF"
+func namespaceManglesIntoName(arg: namespacedT) {
+}