Array.prototype.flat typing is incorrect

[0xCLARITY]

TypeScript Version: 3.9.3, 4.0.0-dev.20200602, Nightly

Search Terms:

• flat
• Array.prototype.flat

Expected behavior:
Given abc = [[1,2]], I expect abc.flat() to equal [1,2].

Actual behavior:
I get the following error: error TS2339: Property 'flat' does not exist on type 'number[][]'.

Related Issues:
Potentially related to #38298

Code

const abc = [[1, 2]]

const xyz = abc.flat()

Output

"use strict";
const abc = [[1, 2]];
const xyz = abc.flat();

Compiler Options

{
  "compilerOptions": {
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictPropertyInitialization": true,
    "strictBindCallApply": true,
    "noImplicitThis": true,
    "noImplicitReturns": true,
    "useDefineForClassFields": false,
    "alwaysStrict": true,
    "allowUnreachableCode": false,
    "allowUnusedLabels": false,
    "downlevelIteration": false,
    "noEmitHelpers": false,
    "noLib": false,
    "noStrictGenericChecks": false,
    "noUnusedLocals": false,
    "noUnusedParameters": false,
    "esModuleInterop": true,
    "preserveConstEnums": false,
    "removeComments": false,
    "skipLibCheck": false,
    "checkJs": false,
    "allowJs": false,
    "declaration": true,
    "experimentalDecorators": false,
    "emitDecoratorMetadata": false,
    "target": "ES2017",
    "module": "ESNext"
  }
}

Playground Link: Provided

[RyanCavanaugh]

Please add to #36554

[RyanCavanaugh]

Actually, cancel that, set target or lib to ES2019 to get flat typings. Your target (in Playground) is set to ES2017 but flat is an ES2019 feature

[0xCLARITY]

Ah, my mistake. Thank you for the help!