Don't mutate literal object types during subtyping

Summary:
Changelog:
[errors] We will now emit new errors in places where an object literal has literal types, but doesn't have an annotation. You can usually fix these errors by adding annotations to object literals.
[lib] Improved libdef for fs.readdir and fs.readdirSync for better overload resolution

This behavior was introduced to catch the following cases:

```
var a = { p: 0 };
var b: { p: number | string }; // a.p <- number | string
b.p = "";
(a.p : number); // at runtime, a.p is a string due to aliasing
```

or, similarly with optional properties:

```
var a = { p: 0 };
var b: { p?: number } = a; // a.p <- optional number
b.p = undefined;
(a.p : number); // at runtime, a.p is void due to aliasing
```

While this does introduce some measure of safety in these examples, it also
introduces order dependence to constraints, which violates a fundamental
assumption of the type checker.

LTI will not require this hacky approximation. Instead, we will rely on
contextual typing to provide precise inference for literal types and require
annotations where context is unavailable.

Removing this behavior now will remove some safety, but unblocks development of
LTI. Part of LTI changes the order in which constraints are visited by the
solver, which causes spurious errors due to this change.

Reviewed By: samwgoldman

Differential Revision: D37501336

fbshipit-source-id: 382de86b99d33e3e7fdc75b2260a405e513a5ec5

lib/node.js

@@ -1085,7 +1085,7 @@ declare module "fs" {
   ): void;
   declare function readdir(
     path: string,
-    options: { encoding?: string, withFileTypes?: true, ... },
+    options: { encoding?: string, withFileTypes: true, ... },
     callback: (err: ?ErrnoError, files: Array<Dirent>) => void
   ): void;
   declare function readdir(
@@ -1098,7 +1098,7 @@ declare module "fs" {
   ): Array<string>;
   declare function readdirSync(
     path: string,
-    options?: string | { encoding?: string, withFileTypes?: true, ... }
+    options?: string | { encoding?: string, withFileTypes: true, ... }
   ): Array<Dirent>;
   declare function close(fd: number, callback: (err: ?ErrnoError) => void): void;
   declare function closeSync(fd: number): void;

src/typing/subtyping_kit.ml

@@ -254,13 +254,12 @@ module Make (Flow : INPUT) : OUTPUT = struct
         in
         match (Context.get_prop cx lflds s, ldict) with
         | (Some lp, _) ->
-          if lit then (
-            (* prop from unaliased LB: check <:, then make exact *)
-            (match (Property.read_t lp, Property.read_t up) with
+          if lit then
+            (* prop from unaliased LB: check <: *)
+            match (Property.read_t lp, Property.read_t up) with
             | (Some lt, Some ut) -> rec_flow cx trace (lt, UseT (use_op, ut))
-            | _ -> ());
-            speculative_object_write cx lflds s up
-          ) else
+            | _ -> ()
+          else
             (* prop from aliased LB *)
             rec_flow_p cx ~trace ~use_op lreason ureason propref (lp, up)
         | (None, Some { key; value; dict_polarity; _ }) when not (is_dictionary_exempt s) ->
@@ -287,12 +286,7 @@ module Make (Flow : INPUT) : OUTPUT = struct
         | _ ->
           (* property doesn't exist in inflowing type *)
           (match up with
-          | Field (_, OptionalT _, _) when lit ->
-            (* if property is marked optional or otherwise has a maybe type,
-               and if inflowing type is a literal (i.e., it is not an
-               annotation), then we add it to the inflowing type as
-               an optional property *)
-            speculative_object_write cx lflds s up
+          | Field (_, OptionalT _, _) when lit -> ()
           | Field (_, OptionalT _, Polarity.Positive)
             when Obj_type.is_exact_or_sealed ureason lflags.obj_kind ->
             rec_flow

tests/new_merge/new_merge.exp

@@ -406,20 +406,20 @@ References:
 
 Error ----------------------------------------------------------------------------------------------------- main.js:85:2
 
-Cannot cast `y19` to object type because undefined [1] is incompatible with number [2] in property `a`.
-[incompatible-cast]
+Cannot cast `y19` to object type because property `a` is missing in object literal [1] but exists in object type [2].
+[prop-missing]
 
    main.js:85:2
    85| (y19: { a: number }); // error undefined ~> number
         ^^^
 
 References:
-   main.js:84:13
-   84| (y19: { a?: number }); // okay
-                   ^^^^^^ [1]
-   main.js:85:12
+   spread.js:10:18
+   10| export const y = { ...t };
+                        ^^^^^^^^ [1]
+   main.js:85:7
    85| (y19: { a: number }); // error undefined ~> number
-                  ^^^^^^ [2]
+             ^^^^^^^^^^^^^ [2]
 
 
 Error ----------------------------------------------------------------------------------------------------- main.js:89:2

tests/object_widening/object_widening.exp

@@ -201,67 +201,101 @@ References:
 
 Error -------------------------------------------------------------------------------------------- type_widening.js:54:2
 
-Cannot cast `b.foo` to number because undefined [1] is incompatible with number [2]. [incompatible-cast]
+Cannot cast `b.foo` to number because possibly missing property `foo` in object literal [1] is incompatible with
+number [2]. [incompatible-cast]
 
    type_widening.js:54:2
    54| (b.foo: number); // Error, foo does not appear in all branches
         ^^^^^
 
 References:
-   type_widening.js:17:24
-   17| type AllOpt = { +foo?: number, +bar?: number, +baz?: number, +qux?: number, ... };
-                              ^^^^^^ [1]
+   type_widening.js:48:23
+   48| const b = spreadExact(y);
+                             ^ [1]
    type_widening.js:54:9
    54| (b.foo: number); // Error, foo does not appear in all branches
                ^^^^^^ [2]
 
 
 Error -------------------------------------------------------------------------------------------- type_widening.js:55:2
 
-Cannot cast `b.bar` to number because undefined [1] is incompatible with number [2]. [incompatible-cast]
+Cannot cast `b.bar` to number because possibly missing property `bar` in object literal [1] is incompatible with
+number [2]. [incompatible-cast]
 
    type_widening.js:55:2
    55| (b.bar: number); // Error, bar does not appear in all branches
         ^^^^^
 
 References:
-   type_widening.js:17:39
-   17| type AllOpt = { +foo?: number, +bar?: number, +baz?: number, +qux?: number, ... };
-                                             ^^^^^^ [1]
+   type_widening.js:48:23
+   48| const b = spreadExact(y);
+                             ^ [1]
    type_widening.js:55:9
    55| (b.bar: number); // Error, bar does not appear in all branches
                ^^^^^^ [2]
 
 
 Error -------------------------------------------------------------------------------------------- type_widening.js:56:2
 
-Cannot cast `b.baz` to number because undefined [1] is incompatible with number [2]. [incompatible-cast]
+Cannot cast `b.baz` to number because possibly missing property `baz` in object literal [1] is incompatible with
+number [2]. [incompatible-cast]
 
    type_widening.js:56:2
    56| (b.baz: number); // Error, baz does not appear in all branches
         ^^^^^
 
 References:
-   type_widening.js:17:54
-   17| type AllOpt = { +foo?: number, +bar?: number, +baz?: number, +qux?: number, ... };
-                                                            ^^^^^^ [1]
+   type_widening.js:48:23
+   48| const b = spreadExact(y);
+                             ^ [1]
+   type_widening.js:56:9
+   56| (b.baz: number); // Error, baz does not appear in all branches
+               ^^^^^^ [2]
+
+
+Error -------------------------------------------------------------------------------------------- type_widening.js:56:2
+
+Cannot cast `b.baz` to number because `void` (due to access of non-existent property `baz`) [1] is incompatible with
+number [2]. [incompatible-cast]
+
+   type_widening.js:56:2
+   56| (b.baz: number); // Error, baz does not appear in all branches
+        ^^^^^ [1]
+
+References:
    type_widening.js:56:9
    56| (b.baz: number); // Error, baz does not appear in all branches
                ^^^^^^ [2]
 
 
 Error -------------------------------------------------------------------------------------------- type_widening.js:57:2
 
-Cannot cast `b.qux` to number because undefined [1] is incompatible with number [2]. [incompatible-cast]
+Cannot cast `b.qux` to number because possibly missing property `qux` in object literal [1] is incompatible with
+number [2]. [incompatible-cast]
 
    type_widening.js:57:2
    57| (b.qux: number); // Error, qux does not appear in all branches
         ^^^^^
 
 References:
-   type_widening.js:17:69
-   17| type AllOpt = { +foo?: number, +bar?: number, +baz?: number, +qux?: number, ... };
-                                                                           ^^^^^^ [1]
+   type_widening.js:48:23
+   48| const b = spreadExact(y);
+                             ^ [1]
+   type_widening.js:57:9
+   57| (b.qux: number); // Error, qux does not appear in all branches
+               ^^^^^^ [2]
+
+
+Error -------------------------------------------------------------------------------------------- type_widening.js:57:2
+
+Cannot cast `b.qux` to number because `void` (due to access of non-existent property `qux`) [1] is incompatible with
+number [2]. [incompatible-cast]
+
+   type_widening.js:57:2
+   57| (b.qux: number); // Error, qux does not appear in all branches
+        ^^^^^ [1]
+
+References:
    type_widening.js:57:9
    57| (b.qux: number); // Error, qux does not appear in all branches
                ^^^^^^ [2]
@@ -806,4 +840,4 @@ References:
 
 
 
-Found 49 errors
+Found 51 errors