[clang][ASTImporter] Fix AST import if anonymous namespaces are merged (#128735)

Fix of a faulty case that is shown in the second of the added
tests (an anonymous namespace is imported that resides in a `extern "C"`
block).

1 files changed, 4 insertions, 3 deletions

diff --git a/clang/lib/AST/ASTImporter.cpp b/clang/lib/AST/ASTImporter.cpp
index eb0e686..b481ad5 100644
--- a/clang/lib/AST/ASTImporter.cpp
+++ b/clang/lib/AST/ASTImporter.cpp
@@ -2628,11 +2628,12 @@ ExpectedDecl ASTNodeImporter::VisitNamespaceDecl(NamespaceDecl *D) {
   if (!Name) {
     // This is an anonymous namespace. Adopt an existing anonymous
     // namespace if we can.
-    // FIXME: Not testable.
-    if (auto *TU = dyn_cast<TranslationUnitDecl>(DC))
+    DeclContext *EnclosingDC = DC->getEnclosingNamespaceContext();
+    if (auto *TU = dyn_cast<TranslationUnitDecl>(EnclosingDC))
       MergeWithNamespace = TU->getAnonymousNamespace();
     else
-      MergeWithNamespace = cast<NamespaceDecl>(DC)->getAnonymousNamespace();
+      MergeWithNamespace =
+          cast<NamespaceDecl>(EnclosingDC)->getAnonymousNamespace();
   } else {
     SmallVector<NamedDecl *, 4> ConflictingDecls;
     auto FoundDecls = Importer.findDeclsInToCtx(DC, Name);