Optional chaining leaks undefined type when using function to narrow type

[gustavohenke]

TypeScript Version: 3.7.2

Search Terms:
Optional chaining

Expected behavior:
Everything compiles successfully.

Actual behavior:
For the x variable:

Argument of type 'number | undefined' is not assignable to parameter of type 'number'.
  Type 'undefined' is not assignable to type 'number'.(2345)

Related Issues:

• Optional chaining, proper infer in type guard #34974
• Type narrowing for optional chaining in OR expression in AND expression #35109

Code

function checkExists(x: null | undefined, msg?: string, ...args: any[]): never;
function checkExists<T>(x: T | null | undefined): T;
function checkExists<T>(x: T | null | undefined): T {
    if (x == null) {
        throw new Error('argument is null');
    }
    return x;
}

type SomeType = {
    prop: number | undefined;
}

function foo(): SomeType | undefined {
    return { prop: 123 };
}

function bar(val: number) {}

// Doesn't work
const val = foo();
const x = checkExists(val?.prop);
bar(x);

// Works
const y = checkExists(val && val.prop);
bar(y);

// Works
const z = checkExists(val?.prop as number | undefined);
bar(z);

Output

"use strict";
var _a, _b;
function checkExists(x) {
    if (x == null) {
        throw new Error('argument is null');
    }
    return x;
}
function foo() {
    return { prop: 123 };
}
function bar(val) { }
const val = foo();
const x = checkExists((_a = val) === null || _a === void 0 ? void 0 : _a.prop);
bar(x);
const y = checkExists(val && val.prop);
bar(y);
const z = checkExists((_b = val) === null || _b === void 0 ? void 0 : _b.prop);
bar(z);

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

[ajafff]

Please try with typescript@next as this issue is already fixed in master

[gustavohenke]

Thanks. Will keep an eye on stable releases.