[C++20] [Modules] Fix may-be incorrect ADL for module local entities (#123931)

Close https://github.com/llvm/llvm-project/issues/123815

See the comments for details. We can't get primary context arbitrarily
since the redecl may have different context and information.

There is a TODO for modules specific case, I'd like to make it after
this PR.

1 files changed, 51 insertions, 1 deletions

diff --git a/clang/lib/Sema/SemaLookup.cpp b/clang/lib/Sema/SemaLookup.cpp
index 2ed8d36..5f8ffa7 100644
--- a/clang/lib/Sema/SemaLookup.cpp
+++ b/clang/lib/Sema/SemaLookup.cpp
@@ -2914,7 +2914,57 @@ static void CollectEnclosingNamespace(Sema::AssociatedNamespaceSet &Namespaces,
   while (!Ctx->isFileContext() || Ctx->isInlineNamespace())
     Ctx = Ctx->getParent();
 
-  Namespaces.insert(Ctx->getPrimaryContext());
+  // Actually it is fine to always do `Namespaces.insert(Ctx);` simply. But it
+  // may cause more allocations in Namespaces and more unnecessary lookups. So
+  // we'd like to insert the representative namespace only.
+  DeclContext *PrimaryCtx = Ctx->getPrimaryContext();
+  Decl *PrimaryD = cast<Decl>(PrimaryCtx);
+  Decl *D = cast<Decl>(Ctx);
+  ASTContext &AST = D->getASTContext();
+
+  // TODO: Technically it is better to insert one namespace per module. e.g.,
+  //
+  // ```
+  // //--- first.cppm
+  // export module first;
+  // namespace ns { ... } // first namespace
+  //
+  // //--- m-partA.cppm
+  // export module m:partA;
+  // import first;
+  //
+  // namespace ns { ... }
+  // namespace ns { ... }
+  //
+  // //--- m-partB.cppm
+  // export module m:partB;
+  // import first;
+  // import :partA;
+  //
+  // namespace ns { ... }
+  // namespace ns { ... }
+  //
+  // ...
+  //
+  // //--- m-partN.cppm
+  // export module m:partN;
+  // import first;
+  // import :partA;
+  // ...
+  // import :part$(N-1);
+  //
+  // namespace ns { ... }
+  // namespace ns { ... }
+  //
+  // consume(ns::any_decl); // the lookup
+  // ```
+  //
+  // We should only insert once for all namespaces in module m.
+  if (D->isInNamedModule() &&
+      !AST.isInSameModule(D->getOwningModule(), PrimaryD->getOwningModule()))
+    Namespaces.insert(Ctx);
+  else
+    Namespaces.insert(PrimaryCtx);
 }
 
 // Add the associated classes and namespaces for argument-dependent