[NFC] [C++20] [Modules] Update comments for handling friend

There is a comment contains a FIXME for the Module TS. And now the
Module TS is merged so we should update the comment. I've checked the
implementation.

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index a294094..e014500 100644 (file)
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -1586,10 +1586,13 @@ void Sema::FilterLookupForScope(LookupResult &R, DeclContext *Ctx, Scope *S,
 /// We've determined that \p New is a redeclaration of \p Old. Check that they
 /// have compatible owning modules.
 bool Sema::CheckRedeclarationModuleOwnership(NamedDecl *New, NamedDecl *Old) {
-  // FIXME: The Modules TS is not clear about how friend declarations are
-  // to be treated. It's not meaningful to have different owning modules for
-  // linkage in redeclarations of the same entity, so for now allow the
-  // redeclaration and change the owning modules to match.
+  // [module.interface]p7:
+  // A declaration is attached to a module as follows:
+  // - If the declaration is a non-dependent friend declaration that nominates a
+  // function with a declarator-id that is a qualified-id or template-id or that
+  // nominates a class other than with an elaborated-type-specifier with neither
+  // a nested-name-specifier nor a simple-template-id, it is attached to the
+  // module to which the friend is attached ([basic.link]).
   if (New->getFriendObjectKind() &&
       Old->getOwningModuleForLinkage() != New->getOwningModuleForLinkage()) {
     New->setLocalOwningModule(Old->getOwningModule());