diff -r b3c65c984210 -r 1091880266e5 src/Pure/Syntax/ast.ML
--- a/src/Pure/Syntax/ast.ML	Sat Sep 04 20:01:43 2021 +0200
+++ b/src/Pure/Syntax/ast.ML	Sat Sep 04 21:25:08 2021 +0200
@@ -158,7 +158,7 @@
           end
       | _ => (ast, []));
   in
-    SOME (mtch head pat Symtab.empty, args) handle NO_MATCH => NONE
+    SOME (Symtab.build (mtch head pat), args) handle NO_MATCH => NONE
   end;