Refinements should be invalidated by side effects

const a:  { a: string | number } = { a: 'foo' };

function foo() {
  a.a = 1;
}

function test(x: { a: string | number }) {
    if (typeof x.a === 'string') {
      foo(); // this invalidates the refinement
      const a: string = x.a.toUpperCase(); // runtime error
    }
}

 test(a);

[vkurchatkin]

I guess if soundness is the goal, then there is no middle ground

[Raynos]

Should some of these soundness features be behind opt in / opt out flags ?

Like maybe in my code base I would be ok with assuming “no side effects in function calls” and some other code bases might really like the advanced checking of “Please invalidate on side effects”