fix crash with inline record type variable

Author: EmileTrotignon

lib/Fmt_ast.ml

@@ -3649,6 +3649,9 @@ and fmt_constructor_arguments ?vars c ctx ~pre = function
       in
       pre $ vars $ typs
   | Pcstr_record (loc, lds) ->
+      let vars =
+        match vars with Some vars -> space_break $ vars | None -> noop
+      in
       let p = Params.get_record_type c.conf in
       let fmt_ld ~first ~last x =
         fmt_if (not first) p.sep_before
@@ -3658,7 +3661,7 @@ and fmt_constructor_arguments ?vars c ctx ~pre = function
             (str " ")
         $ fmt_if (not last) p.sep_after
       in
-      pre
+      pre $ vars
       $ Cmts.fmt c loc ~pro:(break 1 0) ~epi:noop
         @@ wrap p.docked_before p.docked_after
         @@ wrap p.break_before p.break_after

test/passing/tests/record.ml

@@ -89,3 +89,4 @@ let x = A(B).a
 
 let x = (1).a
 
+type 'a t = A : 'b. {a: 'a} -> 'a t