Parameter not matching union type not recognized when passed by reference

[mr-flannery]

TypeScript Version: 3.3.3333, 3.4.2, 3.4.5, 3.5.2, 3.6.0-dev.20190704

Search Terms: union, union + object, union + object + bug

Code

interface A {
  a: string;
}

interface B {
  b: string;
}

const fn = (param: A | B) => param;

const obj = { a: "sdf", c: "sdf" };

fn({ a: "sd" });
fn({ b: "sd" });
fn({ a: "sd", b: "sfg" });
fn({ a: "sdf", c: "sdf" }); // this makes tsc fail
fn(obj); // this does not, but it should

Expected behavior:

tsc reports two errors, one for the last two lines respectively, as the parameters passed are essentially the same.

Actual behavior:

tsc only reports the second to last line as error, not the last one (or differently: if I comment the second to last line out, tsc does not fail, though it should)

Playground Link: https://www.typescriptlang.org/play/index.html#code/JYOwLgpgTgZghgYwgAgILIN4ChnLgLmQGcwpQBzAbiwF8stRJZEUAhTHZAI0JLJCq16CAPYgSyGCGQBeZAAoADnChwAtoXQAfZKwCUsgHzJlqtdSyjxYZCK4ArWZjyEAREQAmMVwBpkCN09vZBoLKXkMF2R3D1cQvWpwyJ5ozziaBKwkqJjfbkCYcnTM7IJUrzyA8uCMxJB5O3tMoA

Thank you very much!

[fatcerberus]

obj is assignable to A | B by way of structural subtyping; in particular, it satisfies A. Types are not closed: the error you do see is caused by passing in a fresh object literal with an excess property, which is checked for to catch typos and similar mistakes; it's not an actual typing violation.

I agree the error message is very misleading, see discussion starting here: #32158 (comment)

Here's a pretty good explanation (IMO) of why fn(obj) is not an error: https://basarat.gitbooks.io/typescript/docs/types/freshness.html

[MartinJohns]

https://github.com/microsoft/TypeScript/wiki/FAQ#what-is-structural-typing

[jcalz]

Handbook documentation for excess property checking

[mr-flannery]

Ah, I wasn't aware of this behavior! Thank you for the quick response!

[fatcerberus]

@mr-flannery Yeah, under pervasive structural typing, it’s the only behavior that makes sense, because otherwise you wouldn’t be able to have subclasses at all.