More special casing for solved tyvars

Summary:
In legacy errors the reasons for tyvar solutions _replace_ the reason for the tyvar. In extended reasons we record the solution to ensure we don't destroy provenance. To recreate the legacy error messages we want to:
1) Propagate solutions through `Flow`s and use the solution reason when we find the corresponding tyvar
2) If a `Flow` doesn't contain a tyvar, use the outermost reason.

This diff special cases the situation where we have a `Solved` reason wrapping a flow which doesn't contain a type variable

Reviewed By: madgen

Differential Revision: D63898873

fbshipit-source-id: fdeb515fdaa64fec728783d6a0ff7e2a92edf964

Author: Michael Thomas

hphp/hack/src/typing/typing_reason.ml

@@ -2052,6 +2052,17 @@ let to_pos : type ph. ph t_ -> Pos_or_decl.t =
   else
     to_raw_pos r
 
+let rec flow_contains_tyvar = function
+  | Flow
+      {
+        from = From_witness_locl (Type_variable_generics _ | Type_variable _);
+        _;
+      }
+  | From_witness_locl (Type_variable_generics _ | Type_variable _) ->
+    true
+  | Flow { into; _ } -> flow_contains_tyvar into
+  | _ -> false
+
 (* Translate a reason to a (pos, string) list, suitable for error_l. This
  * previously returned a string, however the need to return multiple lines with
  * multiple locations meant that it needed to more than convert to a string *)
@@ -2078,8 +2089,9 @@ let rec to_string_help :
   | Prj_one { part = r; _ } ->
     to_string_help prefix solutions r
   (* If we don't have a solution for a type variable use the origin of the flow *)
-  | Flow { from = r; _ } when Tvid.Map.is_empty solutions ->
-    to_string_help prefix solutions r
+  | Flow { from; _ }
+    when Tvid.Map.is_empty solutions || not (flow_contains_tyvar r) ->
+    to_string_help prefix solutions from
   (* otherwise, follow the flow until we reach the type variable *)
   | Flow { from; into; _ } ->
     (match from with