Weaken type checks based on CFA with assertion conditions

Too strict for tests.

cc @ahejlsberg

TypeScript Version: 3.7.x-dev.20191207

Search Terms:

Code

declare function assert(value: any): asserts value;
declare const set: Set<0>;

assert(set.size === 0);
set.add(0);
assert(set.size === 1);

Expected behavior: pass Actual behavior: This condition will always return ‘false’ since the types ‘0’ and ‘1’ have no overlap.(2367) Playground Link: http://www.typescriptlang.org/play/index.html?ssl=1&ssc=1&pln=6&pc=24#code/CYUwxgNghgTiAEAzArgOzAFwJYHtXygGdCQYMAKANygmRAC4DUBPASkaJLMPmtpADcAKFCRYCMHkIZ4JDIwDKIDAB4ADAD5hQzqQpyAdISwAvBAF5L8Na2GGowYORvDdZcoeNn4l8-ACMtkA

Related Issues:

[jcalz]

Let’s imagine that we disable overlap detection. Then what do you expect set.size to be at the end of your example code? I think it would end up looking like this:

declare function guard<T>(x: any, v: T): x is T;
declare const set: Set<0>;
if (!guard(set.size, 0 as const)) throw new Error(); // assert(set.size === 0)
if (!guard(set.size, 1 as const)) throw new Error(); // assert(set.size === 1)
set.size; // <--- never

Unless you want to narrow to never, I suspect you are using asserts functions in places you shouldn’t. Perhaps you want two assert() functions, one which performs CFA narrowing and one which doesn’t:

// keep using this one for your tests
function assert(x: any) {
    if (!x) throw new Error();
}

// use this one for things you actually want to narrow
function assertCFA(x: any): asserts x {
    assert(x);
}

Or, is there some explicit suggestion here for how to modify the behavior of TS assertion functions so that they behave well for your use case without disabling CFA narrowing entirely?

[falsandtru]

@ahejlsberg Can you make your new assertion functions usable for the existing tests? They don’t support some typical test patterns. Otherwise I have to revert the introduction of them.