Fix error handling for nested unions and intersections

Union and intersection errors are handled using a speculative match
process, where an error on one side of the match raises an exception,
which is caught by the speculative match code, which then tries the
otehr side of the match.

If, however, there are nested speculative matches in a given flow, the
innermost one would raise an error too early. This can happen if a type
alias is a union containing union types.

To fix, instead of using a boolean to track whether to log the error or
raise, we use an integer representing the "depth" of the speculative
match.

Before, there was some code to "flatten" unions of unions during the
convertion of annotations to types, but this doesn't work for the case
where the unions are themselves type aliases.

Unfortunately, this change affects error messages around union types,
and in my opinion, the new messages are worse and less helpful than they
used to be. Any ideas how to make this change without making error
messages worse?

Fixes facebook#582

src/typing/flow_js.ml

@@ -98,14 +98,14 @@ let mk_objecttype ?(flags=default_flags) dict map proto = {
 
 (**************************************************************)
 
-(* for now, we do speculative matching by setting this global.
-   it's set to true when trying the arms of intersection or
+(* For now, we do speculative matching by incrementing this global.
+   It's incremented when trying the arms of intersection or
    union types.
-   when it's true, the error-logging function throws instead
+   When it's non-zero, the error-logging function throws instead
    of logging, and the speculative harness catches.
    TODO
  *)
-let throw_on_error = ref false
+let throw_on_error = ref 0
 
 (* we keep a stack of reasons representing the operations
    taking place when flows are performed. the top op reason
@@ -199,7 +199,7 @@ let silent_warnings = false
 exception FlowError of (reason * string) list
 
 let add_output cx level ?(trace_reasons=[]) message_list =
-  if !throw_on_error
+  if !throw_on_error > 0
   then (
     raise (FlowError message_list)
   ) else (
@@ -3569,16 +3569,16 @@ and speculative_flow_error cx trace l u =
   (* save the ops stack, since throws from within __flow will screw it up *)
   let ops = Ops.get () in
   let typeapp_stack = TypeAppExpansion.get () in
-  throw_on_error := true;
+  throw_on_error := !throw_on_error + 1;
   let result =
     try rec_flow cx trace (l, u); false
     with
     | FlowError msgs -> true
     | exn ->
-        throw_on_error := false;
+        throw_on_error := 0;
         raise exn
   in
-  throw_on_error := false;
+  throw_on_error := !throw_on_error - 1;
   TypeAppExpansion.set typeapp_stack;
   (* restore ops stack *)
   Ops.set ops;

src/typing/type_inference_js.ml

@@ -576,12 +576,6 @@ let rec convert cx map = Ast.Type.(function
 
   | loc, Union ts ->
       let ts = List.map (convert cx map) ts in
-      (* "Flatten" out any unions in this union, like
-       * var a: number | (string | bool) *)
-      let ts = List.map (function
-        | UnionT (r, ts) -> ts
-        | t -> [t]) ts in
-      let ts = List.flatten ts in
       UnionT (mk_reason "union type" loc, ts)
 
   | loc, Intersection ts ->

tests/namespace/namespace.exp

@@ -11,7 +11,7 @@ namespace.js:11:28,29: string
 This type is incompatible with
 namespace.js:11:7,12: number
 
-namespace.js:15:16,17: string
+namespace.js:15:14,19: array literal
 This type is incompatible with
 namespace.js:13:12,30: union type
 

tests/promises/promises.exp

@@ -7,13 +7,13 @@ This type is incompatible with
 
 promise.js:21:11,23:4: constructor call
 Error:
-promise.js:22:13,13: number
+[LIB] core.js:350:1,377:1: Promise
 This type is incompatible with
 [LIB] core.js:352:25,38: union type
 
 promise.js:32:13,34:6: constructor call
 Error:
-promise.js:33:15,15: number
+[LIB] core.js:350:1,377:1: Promise
 This type is incompatible with
 [LIB] core.js:352:25,38: union type
 
@@ -31,13 +31,13 @@ This type is incompatible with
 
 promise.js:112:17,34: call of method resolve
 Error:
-promise.js:112:33,33: number
+[LIB] core.js:350:1,377:1: Promise
 This type is incompatible with
 [LIB] core.js:370:32,45: union type
 
 promise.js:118:33,50: call of method resolve
 Error:
-promise.js:118:49,49: number
+[LIB] core.js:350:1,377:1: Promise
 This type is incompatible with
 [LIB] core.js:370:32,45: union type
 
@@ -49,13 +49,13 @@ This type is incompatible with
 
 promise.js:157:32,54: call of method resolve
 Error:
-promise.js:157:48,53: string
+[LIB] core.js:350:1,377:1: Promise
 This type is incompatible with
-[LIB] core.js:370:32,45: union type
+[LIB] core.js:357:33,46: union type
 
 promise.js:165:48,70: call of method resolve
 Error:
-promise.js:165:64,69: string
+[LIB] core.js:350:1,377:1: Promise
 This type is incompatible with
 [LIB] core.js:370:32,45: union type
 
@@ -67,13 +67,13 @@ This type is incompatible with
 
 promise.js:197:33,55: call of method resolve
 Error:
-promise.js:197:49,54: string
+[LIB] core.js:350:1,377:1: Promise
 This type is incompatible with
-[LIB] core.js:370:32,45: union type
+[LIB] core.js:367:34,48: union type
 
 promise.js:205:49,71: call of method resolve
 Error:
-promise.js:205:65,70: string
+[LIB] core.js:350:1,377:1: Promise
 This type is incompatible with
 [LIB] core.js:370:32,45: union type
 

tests/union/union.js

@@ -19,3 +19,11 @@ function corge(b) {
     var x = b ? "" : 0;
     new F().foo(x);
 }
+
+var ok: ('foo' | 'bar') | 'baz' = 'baz';
+
+type FooBar = 'foo' | 'bar';
+type Baz = 'baz';
+type FooBarBaz = FooBar | Baz;
+
+var bad: FooBarBaz = 'baz';