[flow][lib] restrict typing of Array.concat

Summary:
Before, given the declaration
```
declare class $ReadOnlyArray<+T> {
  ...
  concat<S, Item: $ReadOnlyArray<S> | S>(...items: Array<Item>): Array<T | S>;
  ...
}
```
the following forms were errors
```
declare const arr: Array<number>;
arr.concat();
arr.concat() as Array<number>;
```
* The first due to under-constrained `S`
* The second due to inferring a bad bound (`mixed`) which is incompatible with `number`

This change makes `.concat` behave better when called without arguments.

Changelog: [fix] improved definition of `Array.concat()`.

Reviewed By: gkz

Differential Revision: D67421211

fbshipit-source-id: b48516259e80e94134f07320504e0e4dffd35d79

lib/core.js

@@ -713,7 +713,7 @@ declare class $ReadOnlyArray<+T> {
      * Combines two or more arrays.
      * @param items Additional items to add to the end of array1.
      */
-    concat<S, Item: $ReadOnlyArray<S> | S>(...items: Array<Item>): Array<T | S>;
+    concat<S = T>(...items: Array<$ReadOnlyArray<S> | S>): Array<T | S>;
     /**
      * Returns an iterable of key, value pairs for every entry in the array
      */

tests/arraylib/array_lib.js

@@ -24,6 +24,9 @@ var k: Array<number> = h.concat(h);
 var l: Array<number> = h.concat(1, 2, 3);
 var m: Array<number | string> = h.concat('a', 'b', 'c');
 var n: Array<number> = h.concat('a', 'b', 'c'); // Error
+var o = h.concat();
+o as Array<number>; // OK
+var p: Array<number> = h.concat(); // OK
 
 function reduce_test() {
   /* Adapted from the following source:

tests/arraylib/arraylib.exp

@@ -95,83 +95,83 @@ References:
                     ^^^^^^ [2]
 
 
-Error ----------------------------------------------------------------------------------------------- array_lib.js:55:29
+Error ----------------------------------------------------------------------------------------------- array_lib.js:58:29
 
 Cannot use operator `*` with operands string [1] and number [2] [unsafe-arithmetic]
 
-   array_lib.js:55:29
-     55|   [''].reduce((acc, str) => acc * str.length); // error, string ~> number
+   array_lib.js:58:29
+     58|   [''].reduce((acc, str) => acc * str.length); // error, string ~> number
                                      ^^^^^^^^^^^^^^^^
 
 References:
-   array_lib.js:55:4
-     55|   [''].reduce((acc, str) => acc * str.length); // error, string ~> number
+   array_lib.js:58:4
+     58|   [''].reduce((acc, str) => acc * str.length); // error, string ~> number
             ^^ [1]
    <BUILTINS>/core.js:1365:13
    1365|     length: number;
                      ^^^^^^ [2]
 
 
-Error ----------------------------------------------------------------------------------------------- array_lib.js:56:34
+Error ----------------------------------------------------------------------------------------------- array_lib.js:59:34
 
 Cannot use operator `*` with operands string [1] and number [2] [unsafe-arithmetic]
 
-   array_lib.js:56:34
-     56|   [''].reduceRight((acc, str) => acc * str.length); // error, string ~> number
+   array_lib.js:59:34
+     59|   [''].reduceRight((acc, str) => acc * str.length); // error, string ~> number
                                           ^^^^^^^^^^^^^^^^
 
 References:
-   array_lib.js:56:4
-     56|   [''].reduceRight((acc, str) => acc * str.length); // error, string ~> number
+   array_lib.js:59:4
+     59|   [''].reduceRight((acc, str) => acc * str.length); // error, string ~> number
             ^^ [1]
    <BUILTINS>/core.js:1365:13
    1365|     length: number;
                      ^^^^^^ [2]
 
 
-Error ------------------------------------------------------------------------------------------------ array_lib.js:68:3
+Error ------------------------------------------------------------------------------------------------ array_lib.js:71:3
 
 Cannot cast `Array.from(...)` to array type because string [1] is incompatible with empty [2] in array element.
 [incompatible-cast]
 
-   array_lib.js:68:3
-     68|   Array.from('abcd') as Array<empty>; // ERROR
+   array_lib.js:71:3
+     71|   Array.from('abcd') as Array<empty>; // ERROR
            ^^^^^^^^^^^^^^^^^^
 
 References:
    <BUILTINS>/core.js:1139:37
    1139|     static from(str: string): Array<string>;
                                              ^^^^^^ [1]
-   array_lib.js:68:31
-     68|   Array.from('abcd') as Array<empty>; // ERROR
+   array_lib.js:71:31
+     71|   Array.from('abcd') as Array<empty>; // ERROR
                                        ^^^^^ [2]
 
 
-Error ----------------------------------------------------------------------------------------------- array_lib.js:83:57
+Error ----------------------------------------------------------------------------------------------- array_lib.js:86:57
 
 Cannot call `Array.of` because number [1] is incompatible with string [2] in array element. [incompatible-call]
 
-   array_lib.js:83:57
-   83|   var incompatibleTypeNotOkay: Array<string> = Array.of(1, 2);
+   array_lib.js:86:57
+   86|   var incompatibleTypeNotOkay: Array<string> = Array.of(1, 2);
                                                                ^ [1]
 
 References:
-   array_lib.js:83:38
-   83|   var incompatibleTypeNotOkay: Array<string> = Array.of(1, 2);
+   array_lib.js:86:38
+   86|   var incompatibleTypeNotOkay: Array<string> = Array.of(1, 2);
                                             ^^^^^^ [2]
 
 
-Error ----------------------------------------------------------------------------------------------- array_lib.js:83:60
+Error ----------------------------------------------------------------------------------------------- array_lib.js:86:60
 
 Cannot call `Array.of` because number [1] is incompatible with string [2] in array element. [incompatible-call]
 
-   array_lib.js:83:60
-   83|   var incompatibleTypeNotOkay: Array<string> = Array.of(1, 2);
+   array_lib.js:86:60
+   86|   var incompatibleTypeNotOkay: Array<string> = Array.of(1, 2);
                                                                   ^ [1]
 
 References:
-   array_lib.js:83:38
-   83|   var incompatibleTypeNotOkay: Array<string> = Array.of(1, 2);
+   array_lib.js:86:38
+   86|   var incompatibleTypeNotOkay: Array<string> = Array.of(1, 2);
                                             ^^^^^^ [2]
 
 

tests/autocomplete_from_h_to_l/autocomplete_from_h_to_l.exp

@@ -2345,7 +2345,7 @@ Flags: --pretty
     {"name":"?.at","type":"void | ((index: number) => T | void)"},
     {
       "name":"?.concat",
-      "type":"void | (<S, Item: $ReadOnlyArray<S> | S>(...items: Array<Item>) => Array<T | S>)"
+      "type":"void | (<S = T>(...items: Array<$ReadOnlyArray<S> | S>) => Array<T | S>)"
     },
     {
       "name":"?.copyWithin",
@@ -2457,7 +2457,7 @@ Flags: --pretty
     {"name":"at","type":"(index: number) => T | void"},
     {
       "name":"concat",
-      "type":"<S, Item: $ReadOnlyArray<S> | S>(...items: Array<Item>) => Array<T | S>"
+      "type":"<S = T>(...items: Array<$ReadOnlyArray<S> | S>) => Array<T | S>"
     },
     {
       "name":"copyWithin",

tests/rec/rec.exp

@@ -393,8 +393,8 @@ defined. [method-unbinding]
 
 References:
    <BUILTINS>/core.js:716:5
-   716|     concat<S, Item: $ReadOnlyArray<S> | S>(...items: Array<Item>): Array<T | S>;
-            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ [2]
+   716|     concat<S = T>(...items: Array<$ReadOnlyArray<S> | S>): Array<T | S>;
+            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ [2]
 
 
 Error ----------------------------------------------------------------------------------------------- this_subst.js:1:53