False branch of in-operator type guard for non-union intersection is not never

[pizzacat83]

TypeScript Version: Nightly

Search Terms: intersection guard, intersection never

Expected behavior: In the first function, the type of obj in the else block is never

Actual behavior: In the first function, the type of obj in the else block is { str: string } & { num: number }, the same as in the if block.

Related Issues: I didn't find any.

Code

(obj: { str: string } & { num: number }) => {
    // Non-Union Intersection type doesn't work as expected.
    if ('str' in obj) {
        const s: string = obj.str;
        // obj: { str: string } & { num: number }
    } else {
        const _exhaustiveCheck: never = obj;
        // Expected: No error. obj: never
        // Actual: Type '{ str: string; } & { num: number; }' is not assignable to type 'never'.
    }
};

(obj: { str: string; num: number }) => {
    // Non-intersection type works as expected.
    if ('str' in obj) {
        const s: string = obj.str;
        // obj: { str: string; num: number }
    } else {
        const _exhaustiveCheck: never = obj;
        // No error. obj: never
    }
};

(obj: { str: string } & { num: number } | { bool: boolean }) => {
    // Union of intersection works as expected.
    if ('str' in obj) {
        const s: string = obj.str;
        // obj: { str: string; num: number }
    } else {
        const proveIsBool: boolean = obj.bool;
        // No error. obj: { bool: boolean }
    }
};

Output

"use strict";
(obj) => {
    // Non-Union Intersection type doesn't work as expected.
    if ('str' in obj) {
        const s = obj.str;
        // obj: { str: string } & { num: number }
    }
    else {
        const _exhaustiveCheck = obj;
        // Expected: No error. obj: never
        // Actual: Type '{ str: string; } & { num: number; }' is not assignable to type 'never'.
    }
};
(obj) => {
    // Non-intersection type works as expected.
    if ('str' in obj) {
        const s = obj.str;
        // obj: { str: string; num: number }
    }
    else {
        const _exhaustiveCheck = obj;
        // No error. obj: never
    }
};
(obj) => {
    // Union of intersection works as expected.
    if ('str' in obj) {
        const s = obj.str;
        // obj: { str: string; num: number }
    }
    else {
        const proveIsBool = obj.bool;
        // No error. obj: { bool: boolean }
    }
};

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]

Reopening due to #38168