Delay the check that detects types variables occurring in patterns until...

Delay the check that detects types variables occurring in patterns until type-checking time (this allows us to have access to delta and thus know the monomorphic variables, which are allowed)

typing/typer.ml

@@ -779,10 +779,6 @@ and branches env b =
     cur_branch := [];
     let ploc = p.loc in
     let p = pat env p in
-    let () =
-      let t = Types.descr (Patterns.accept p) in
-      if not (Types.no_var t) then error ploc "Type variables cannot occur in patterns"
-    in
     let fvp = Patterns.fv p in
     let (fv2,e) = expr (enter_values_dummy fvp env) noloc e in
     let br_loc = merge_loc ploc e.Typed.exp_loc in
@@ -1264,7 +1260,8 @@ and branches_aux loc env targ tres constr precise = function
   | b :: rem ->
     let p = b.br_pat in
     let acc = Types.descr (Patterns.accept p) in
-
+    if not (Var.Set.subset (Types.all_vars acc) env.delta) then
+        error b.br_loc "Generalized type variables cannot occur in patterns";
     let targ' = Types.cap targ acc in
     if Types.is_empty targ' then
         (* this branch cannot be selected: we ignore it *)