Better usage of never return types

[antialize]

The control flow analysis with never return types could be improved.

Consider:

function test1_func() : never {
    throw Error("E");
}

function test1(value: "cat" | 5) {
    // test1_func never returns so there is no exit from the if code path, 
    // therefore we can assume that value is "cat" after the if.
    if (value == 5) test1_func();
    return value.length; 
}

Since test1_func never returns I would expect the type of value at the return statement to be narrowed to "cat".

Consider:

function test2_func()  {
    throw Error("E");
}

I would expect the return type to be deduced as never instead of void.

Finally with a bit better analysis an assert function could be implemented

function test3_func(value:true): void;
function test3_func(value:false): never;
function test3_func(value:boolean) {
    if (!value) throw Error("E");
}
function test3(value:number|null) {
    test3_func(value != null);
    return value + 5;
}

Here since the false value variant never returns, it can be deduced that value must be a number after the call to test3_func.

I though about implementing this my self, but I have no idea of where in the somewhat long cheker.ts.

[DanielRosenwasser]

Looks related to #8655.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.