error TS2339: Property 'length' does not exist on type 'never'

[doberkofler]

TypeScript Version: 3.6.2

Search Terms:

Code

function getValue(): string {
	return '1';
}

function isNumber(value: any): value is number {
	return typeof value === 'number' && !Number.isNaN(value) && Number.isFinite(value);
}

function main() {
	let value = getValue();

	if (isNumber(value) && value.length === 4) {
		value++;
	}
}

main();

Expected behavior:
The compare in the line if (isNumber(value) && value.length === 4) in the above example would always fail but did compile without type error in TypeScript 3.5.3.

Actual behavior:
In TypeScript 3.6.2 the line generates error TS2339: Property 'length' does not exist on type 'never' and I do not understand why this would be a type error?

Playground Link:

Related Issues:

[jack-williams]

This is caused by #31838 which is marked as a breaking change.

In 3.5 the type of value after the type guard isNumber is string & number which is semantically equivalent to never but has an apparent type with property length.

In 3.6 the type of value after the type guard isNumber is eagerly reduced to never which does not have the property length.