A custom type guard produces unexpected behavior inside a generic function

[mckravchyk]

TypeScript Version: 4.1.0-insiders.20201007

Search Terms:
type guard generics

Code

interface IndexedObject {
    [key: string]: any
}

/**
 * Check if the value is a "plain" JavaScript object initialized with { } (not instance of any class)
 * 
 * In addition to the runtime check, this function should assert that the 
 * argument matches IndexedObject interface
 * 
 */
const isPlainObject = (value: any): value is IndexedObject => (
    typeof value === 'object'
    && value !== null
    && value.constructor === Object
    && Object.getPrototypeOf(value) === Object.prototype
);

function genericFunction<T>(value: T): T {
    if (isPlainObject(value)) {
        type x = typeof value; // T - not quite right
        return value;
    }

    type x = typeof value; // "never"

    return value;
}

function normalFunction(value: any): void {
    if (isPlainObject(value)) {
        type x = typeof value; // IndexedObject - ok
    }

    type x = typeof value; // any - ok
}

Expected behavior:
Inside genericFunction() in the if block, the type of value should be T & IndexedObject. After the if block, the type of value should be x

Actual behavior:
Inside the genericFunction(), in the if block, type of value is T, and after the if block - type of value is never. It works ok in a non-generic function

Playground Link: https://www.typescriptlang.org/play?ts=4.1.0-insiders.20201007#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-DUYEFeagmrMUA6FcFUkFRJJRZL49JugrVZrdm9hmgdmkvf7g5FEGdo9d7o62SgFxMAbdQZAmYscNYccMgWA61z9CTGRTwPTjzAl9NOZ7hMLxaQtO5ZRpW1aoncGKfsg9aBEwI5rv+E6AaWM5bmBNalDBRTwRIQA

Comment
I am aware that this is not an ideal use case. "plain object" as I defined it, is not strictly tied to IndexedObject interface - which might cause unexpected problems with types. I have decided to not use a type guard for this function, nevertheless - I wanted to submit this as it seems to be a bug. I could not reproduce a more "generic" example.

[andrewbranch]

Duplicate of #4742

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.

[mckravchyk]

I have just tested it in 5.0.2 an it works correctly now.