From 7ce2344e5579fb13a8dab997c62e7af9538c84b1 Mon Sep 17 00:00:00 2001
From: Justin Grant <justingrant@users.noreply.github.com>
Date: Mon, 16 Aug 2021 14:14:15 -0700
Subject: [PATCH] Allow mixed args to string.prototype.concat()

According to MDN docs for the `concat()` method of the `String` type:

> If the arguments are not of the type string, they are converted to string
> values before concatenating.

Passing numbers, objects, etc to `String.prototype.concat` is is valid JS
behavior but currently causes an error in Flow. It also may be blocking
facebook/react#22064.

This commit changes the arg type: `Array<string>` -> `Array<mixed>`.
---
 lib/core.js                         | 6 ++++--
 newtests/autocomplete/test.js       | 2 +-
 tests/autocomplete/autocomplete.exp | 4 ++--
 3 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/lib/core.js b/lib/core.js
index 993ec5b8422..024a51f0dbf 100644
--- a/lib/core.js
+++ b/lib/core.js
@@ -1103,9 +1103,11 @@ declare class String {
     codePointAt(index: number): number;
     /**
      * Returns a string that contains the concatenation of two or more strings.
-     * @param strings The strings to append to the end of the string.
+     * If the arguments are not of the type string, they are converted to string values before
+     * concatenating.
+     * @param values The values to append to the end of the string.
      */
-    concat(...strings: Array<string>): string;
+    concat(...values: Array<mixed>): string;
     constructor(value?: mixed): void;
     /**
      * Returns true if the sequence of elements of searchString converted to a String is the
diff --git a/newtests/autocomplete/test.js b/newtests/autocomplete/test.js
index d06511a62f4..cba395fa27d 100644
--- a/newtests/autocomplete/test.js
+++ b/newtests/autocomplete/test.js
@@ -158,7 +158,7 @@ export default (suite(({addFile, flowCmd}) => [
              },
              {
                "name": "concat",
-               "type": "(...strings: Array<string>) => string"
+               "type": "(...values: Array<mixed>) => string"
              },
              {
                "name": "endsWith",
diff --git a/tests/autocomplete/autocomplete.exp b/tests/autocomplete/autocomplete.exp
index 244c164beca..562e0dd1e78 100644
--- a/tests/autocomplete/autocomplete.exp
+++ b/tests/autocomplete/autocomplete.exp
@@ -41,7 +41,7 @@ Flags: --pretty
     {"name":"charAt","type":"(pos: number) => string"},
     {"name":"charCodeAt","type":"(index: number) => number"},
     {"name":"codePointAt","type":"(index: number) => number"},
-    {"name":"concat","type":"(...strings: Array<string>) => string"},
+    {"name":"concat","type":"(...values: Array<mixed>) => string"},
     {
       "name":"endsWith",
       "type":"(searchString: string, position?: number) => boolean"
@@ -389,7 +389,7 @@ Flags: --pretty
     {"name":"charAt","type":"(pos: number) => string"},
     {"name":"charCodeAt","type":"(index: number) => number"},
     {"name":"codePointAt","type":"(index: number) => number"},
-    {"name":"concat","type":"(...strings: Array<string>) => string"},
+    {"name":"concat","type":"(...values: Array<mixed>) => string"},
     {
       "name":"endsWith",
       "type":"(searchString: string, position?: number) => boolean"