diff -r 276b07a1b5f5 -r a90ab1ea6458 src/HOL/Import/import_data.ML
--- a/src/HOL/Import/import_data.ML	Sun Aug 04 13:14:33 2024 +0200
+++ b/src/HOL/Import/import_data.ML	Sun Aug 04 13:24:54 2024 +0200
@@ -90,7 +90,7 @@
     val (absn, _) = dest_Const abst
     val (repn, _) = dest_Const rept
     val nty = domain_type (fastype_of rept)
-    val ntyn = fst (dest_Type nty)
+    val ntyn = dest_Type_name nty
     val thy2 = add_typ_map tyname ntyn thy
     val thy3 = add_const_map absname absn thy2
     val thy4 = add_const_map repname repn thy3