diff -r fdde6af19ba7 -r 808d940fa838 src/Pure/Syntax/mixfix.ML
--- a/src/Pure/Syntax/mixfix.ML	Tue Oct 22 12:52:25 2024 +0200
+++ b/src/Pure/Syntax/mixfix.ML	Tue Oct 22 13:39:24 2024 +0200
@@ -167,7 +167,11 @@
 (* binder notation *)
 
 val binder_stamp = stamp ();
-val binder_name = perhaps (try Lexicon.unmark_const) #> Lexicon.mark_binder;
+
+fun binder_name c =
+  (if Lexicon.is_const c then Lexicon.unmark_const c
+   else if Lexicon.is_fixed c then Lexicon.unmark_fixed c
+   else c) |> Lexicon.mark_binder;
 
 val binder_bg = Symbol_Pos.explode0 "(";
 val binder_en = Symbol_Pos.explode0 "_./ _)";