[spreads] Allow primitive spreads

Summary:
Spreading numbers, strings, and bools all result in `{||}`. We can model that instead of saying that those aren't objects, which helps out RN with some of the patterns they like to use.

For example:
```
const styles = {...(showBlue() && {backgroundColor: 'blue'}), color: 'red'};
```
should pass type checking and is a very interesting + reasonable pattern!

object_kit handles a number of other object operations that are not spread, but I haven't verified that we have the correct behavior when we use this new rule in those operations. Until I verify those codepaths, I'm only turning this on for spreads.

Reviewed By: gabelevi

Differential Revision: D18270818

fbshipit-source-id: a4d1f4595cc9adaf8a81393f254d46457e3df1b3

src/typing/object_kit.ml

@@ -1252,6 +1252,20 @@ module Kit (Flow : Flow_common.S) : OBJECT = struct
           let flags = { frozen = true; sealed = Sealed; exact = true } in
           let x = Nel.one { Object.reason; props = SMap.empty; dict = None; flags } in
           resolved cx trace use_op reason resolve_tool tool tout x
+        (* TODO(jmbrown): Investigate if these cases can be used for ReactConfig/ObjecRep/Rest.
+         * In principle, we should be able to use it for Rest, but right now
+         * `const {x, ...y} = 3;` tries to get `x` from Number.
+         * They don't make sense with $ReadOnly's semantics, since $ReadOnly doesn't model
+         * copying/spreading an object. *)
+        | DefT (_, _, (StrT _ | NumT _ | BoolT _))
+          when match tool with
+               | ObjectWiden _
+               | Spread _ ->
+                 true
+               | _ -> false ->
+          let flags = { frozen = true; sealed = Sealed; exact = true } in
+          let x = Nel.one { Object.reason; props = SMap.empty; dict = None; flags } in
+          resolved cx trace use_op reason resolve_tool tool tout x
         (* mixed is treated as {[string]: mixed} except in type spread, where it's treated as
          * {}. Any JavaScript value may be treated as an object and so this is safe.
          *

tests/new_spread/new_spread.exp

@@ -964,12 +964,18 @@ References:
                ^^^^^^ [2]
 
 
-Error ---------------------------------------------------------------------------------------------- type_string.js:1:15
+Error ----------------------------------------------------------------------------------------------- type_string.js:2:2
 
-string literal `foo` [1] is not an object.
+Cannot cast `"foo"` to `O1` because string [1] is incompatible with `O1` [2].
 
-   1| type O1 = {..."foo"}; // error
-                    ^^^^^ [1]
+   type_string.js:2:2
+   2| ("foo": O1); // Error
+       ^^^^^ [1]
+
+References:
+   type_string.js:2:9
+   2| ("foo": O1); // Error
+              ^^ [2]
 
 
 Error ------------------------------------------------------------------------------------------------ type_union.js:9:2

tests/new_spread/type_string.js

@@ -1,2 +1,2 @@
-type O1 = {..."foo"}; // error
-("foo": O1);
+type O1 = {..."foo"}; // No error
+("foo": O1); // Error

tests/objkit_primitives/objkit_primitives.exp

@@ -0,0 +1 @@
+Found 0 errors

tests/objkit_primitives/test.js

@@ -0,0 +1,14 @@
+//@flow
+const a: {||} = {...3};
+const b: {||} = {...(3: 3)};
+const c: {||} = {...''};
+const d: {||} = {...('': '')};
+const e: {||} = {...false};
+const f: {||} = {...(false: false)};
+const g: {||} = {...null};
+const h: {||} = {...(null: null)};
+const i: {||} = {...undefined};
+const j: {||} = {...(undefined: void)};
+
+declare function showBlue(): boolean;
+const styles = {...(showBlue() && {backgroundColor: 'blue'}), color: 'red'};