[clang] [clang] Add fixit for using declaration with a (qualified) namespace (PR #94762)

[via cfe-commits]

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@@ -13080,6 +13080,9 @@ NamedDecl *Sema::BuildUsingDeclaration(
   if (R.getAsSingle<NamespaceDecl>()) {
     Diag(IdentLoc, diag::err_using_decl_can_not_refer_to_namespace)
       << SS.getRange();
+    // Suggest using 'using namespace ...' instead.
+    Diag(SS.getBeginLoc(), diag::note_namespace_using_decl)
+      << FixItHint::CreateInsertion(SS.getBeginLoc(), "namespace ");
     return BuildInvalid();
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zmodem wrote:

I suppose another exercise for our future selves could be to recover as if the user typed "using namespace"

https://github.com/llvm/llvm-project/pull/94762