[flow] consider RArrayPatternRestProp fresh

Summary:
Before this diff,
```
declare var arr :  [number];
[...arr] = [1 as 1];
```
was an error. The reason has to do with the type we would infer for an array type emerging from the rest operation. This type was checked against the defined type of `arr`.

The former type was an array type with `1` as its element, and `RArrayPatternRestProp` as the reason on the array.

`RArrayPatternRestProp` is not a "literal" reason, meaning that it will not short-circuit an Array-to-Array check to a co-variant check on their element types.

The invariant check the follows would cause an error on the side of `number <: 1`.

This diff fixes this by treating the inferred type as a freshly generated array literal, which is consistent with other source of array literal freshness. Specifically, it does so by considering the relevant reason `RArrayPatternRestProp` a literal reason.

Changelog: [fix] fixed a bug that caused spurious errors on rest array assignments (e.g. [try-Flow](https://flow.org/try/#1N4Igxg9gdgZglgcxALlAIwIZoKYBsD6uEEAztvhgE6UYCe+JADpdhgCYowa5kA0I2KAFcAtiRQAXSkOz9sADwxgJ+NPTbYuQ3BMnTZA+Y2yU4IwRO4A6SFBIrGVDGM7c+h46fNRLuKxJIGWh8MeT0ZfhYlCStpHzNsFBAMIQkIEQwJODAQfiEyfBE4eWw2fDgofDBMsAALfAA3KjgsXGxxZC4eAw0G-GhcWn9aY3wWZldu-g1mbGqJUoBaCRHEzrcDEgBrbAk62kXhXFxJ923d-cPRHEpTgyEoMDaqZdW7vKgoOfaSKgOKpqmDA+d4gB5fMA-P6LCCMLLQbiLOoYCqgh6-GDYRYIXYLSgkRZkCR4jpddwPfJLZjpOBkO4AX34kA0SQ0Tyo2AABIDOVRKMhOQBtYQiG4AH3spigCAAugBuAA6UEFVlVfJlnIAvEKAIzyzkAegNnIA7nBjpyoBAJJycLyoJyTJQIJR7bROSIXdglbkQA0TCQ4NAkg0AAxWABMADYdZGQPSgA)).

Reviewed By: SamChou19815

Differential Revision: D69941196

fbshipit-source-id: 0fc9ca8e2bb5c0d30c4e3c1a29372fd5961b2337

src/typing/subtyping_kit.ml

@@ -2106,7 +2106,8 @@ module Make (Flow : INPUT) : OUTPUT = struct
         | RArrayLit
         | REmptyArrayLit
         | RRestArrayLit _
-        | RReactChildren ->
+        | RReactChildren
+        | RArrayPatternRestProp ->
           true
         | _ -> false
       in

tests/destructuring/array_rest.js

@@ -10,3 +10,7 @@ let d = zs[1]; // run off the end
 (d: void); // error: number|string|boolean ~> void
 
 let [...e] = 0;
+
+declare var arr :  [number|string];
+[...arr] = [1]; // okay
+[...arr] = [true]; // error boolean ~> number|string

tests/destructuring/destructuring.exp

@@ -207,6 +207,28 @@ References:
                     ^ [1]
 
 
+Error ---------------------------------------------------------------------------------------------- array_rest.js:16:12
+
+Cannot assign array literal to `arr` because in index 0: [incompatible-type]
+ - Either boolean [1] is incompatible with number [2].
+ - Or boolean [1] is incompatible with string [3].
+
+   array_rest.js:16:12
+   16| [...arr] = [true]; // error boolean ~> number|string
+                  ^^^^^^
+
+References:
+   array_rest.js:16:13
+   16| [...arr] = [true]; // error boolean ~> number|string
+                   ^^^^ [1]
+   array_rest.js:14:21
+   14| declare var arr :  [number|string];
+                           ^^^^^^ [2]
+   array_rest.js:14:28
+   14| declare var arr :  [number|string];
+                                  ^^^^^^ [3]
+
+
 Error -------------------------------------------------------------------------------------------------- computed.js:2:2
 
 Cannot cast `val1` to undefined because string [1] is incompatible with undefined [2]. [incompatible-cast]
@@ -1581,7 +1603,7 @@ References:
 
 
 
-Found 111 errors
+Found 112 errors
 
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