here's a full example. If you do this: function test(p: {k: false | 'x' | 'y' | undefined}) { // do stuff } const p = {k: 'x'} // inferred

here's a full example. If you do this: function test(p: {k: false | 'x' | 'y' | undefined}) { // do stuff } const p = {k: 'x'} // inferred type: {k: string} //... test(p); // type error : Type 'string' is not assignable to type 'false | "x" | "y" | undefined' The problem is that the type of p.k is inferred to `string` not `x`. The compiler can't know this is fine because you could call `foo(p)` in between the declaration and the `test` call, that mutates p.k. If you add `as const` or an explicit type to p it is fine. (I think this is what the GGP was referring to)