Union with a mapped string template results in an unexpected type

[cboar]

Bug Report

🔎 Search Terms

union template string literal unexpected behavior

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about unions leading to unexpected behavior.

⏯ Playground Link

Playground link with relevant code

💻 Code

type A = { [P in `/${string}`]: string };
type B = { [P in 'one' | 'two' | 'three']: number };
type C = A | B;

// should not allow partial! But it is accepted, why?
const bug1: C = { one: 1 };
// should not be able to mix A and B! But it is accepted, why?
const bug2: C = { one: 1, '/hello': 'world' };

🙁 Actual behavior

Observe the mapped type A that uses a string template as an index, allowing only specific strings to be used as a key. When this type is used as part of a union, the resulting type exhibits behavior I found strange.

🙂 Expected behavior

I would expect the type assignment for bug1 and bug2 to not be permitted.

[MartinJohns]

Duplicate of #20863. Both your object literals are compatible with the type A, so assigning is permitted.

[cboar]

Thanks, I wasn't aware of this fact: Unions without discriminant properties do not perform excess property checking.