[flow] print Forced FullyResolved tvars in Debug_js

Summary:
With this change Debug_js will attempt to print FullyResolved tvars that have already been forced.

Changelog: [internal]

Reviewed By: SamChou19815

Differential Revision: D55496056

fbshipit-source-id: 080d7819dc700bdee5d777770d22428c719f6e29

src/typing/debug_js.ml

@@ -1041,7 +1041,13 @@ and dump_tvar_ (depth, tvars) cx id =
         | Goto goto -> spf "%d, Goto %d" id goto.parent
         | Root { constraints = Resolved t; _ } ->
           spf "%d, Resolved %s" id (dump_t_ (depth - 1, stack) cx t)
-        | Root { constraints = FullyResolved _; _ } -> spf "%d, FullyResolved" id
+        | Root { constraints = FullyResolved s; _ } ->
+          let payload =
+            match ForcingState.get_forced s with
+            | None -> "unevaluated"
+            | Some t -> dump_t_ (depth - 1, stack) cx t
+          in
+          spf "%d, FullyResolved %s" id payload
         | Root { constraints = Unresolved { lower; upper; _ }; _ } ->
           if lower = TypeMap.empty && upper = UseTypeMap.empty then
             spf "%d" id

src/typing/type.ml

@@ -3238,6 +3238,8 @@ module Constraint = struct
 
     val force : on_error:(Reason.t -> TypeTerm.t) -> t -> TypeTerm.t
 
+    val get_forced : t -> TypeTerm.t option
+
     val map : f:(TypeTerm.t -> TypeTerm.t) -> t -> t
   end = struct
     type state =
@@ -3269,6 +3271,13 @@ module Constraint = struct
         s.state <- Forced;
         t
 
+    let get_forced s =
+      match s.state with
+      | Unforced
+      | Forcing ->
+        None
+      | Forced -> Some (Lazy.force_val s.valid)
+
     let map ~f s = { valid = Lazy.map f s.valid; error_reason = s.error_reason; state = Unforced }
   end