Union types in generics do not correctly check types.

[jespertheend]

Bug Report

🔎 Search Terms

union generic implicit

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about generic

⏯ Playground Link

Playground link with relevant code

💻 Code

interface TwoSameTypes<T> {
  a: T;
  b: T;
}

// @ts-expect-error
const x : TwoSameTypes<Number | String> = {
  a: 1,
  b: "string", // <-- This does not give an error, because `b` has the type `String | Number`
}

🙁 Actual behavior

No error happens

🙂 Expected behavior

I would expect an error for the value of x because both a and b are expected to have the same type, but they don't in this case.

For the record, and as can be seen from the playground link, when doing TwoSameTypes<Number> | TwoSameTypes<String> instead of TwoSameTypes<Number | String> we do end up with the desired behaviour. But I think these two different ways of defining are equivalent? So it seems like the latter way of doing it should work as well.

Furthermore, here's demo of a use case for this behaviour. I would like to use keyof ValueMap as the generic type, so I don't have to type every possible enum twice.

Apologies in advance if this is a duplicate, I wasn't able to find the right keywords to describe this behaviour.

[nmain]

a and b do have the same type though; and that type is number | string. TwoSameTypes<number> | TwoSameTypes<string> and TwoSameTypes<number | string> are not equivalent, and this is the difference.

You can get the desired behavior here with distributive conditional types: https://www.typescriptlang.org/play?ts=4.4.4#code/JYOwLgpgTgZghgYwgAgGpwDYFcIFk4AOyA3gFACQIWAtgFzIByNARtANwUDOYU9Ayj1ABzDgF9SEsAE8CKAGIB7BQB4AKgD5kAXmSrkEAB6QQAE07IA1hCkKYaTDnxEA-CQrTZ9VR3IA3TPTo2HiEANqqALpiyPQgEL7sEgD0ScigkLCI8kpq+kYQpuZWNnZBjoSaZCnI5B4QXhzVfgH2wU7hUaTV4qR1yLgQYAAWCiYAgtrIiioARFTUM5oAPlM5M9xQixx9A8OjAEKT08pzNDPIK+s8W70yKLsjJgDCRznFtq3lBOocXakAAmBOABaQyyBBgUFQKAKKCkBAKEDcZAGGL9QaPCY6Mi1O70U4LAA0FH8GHx0BhUAAhDNCchqspgcDdENgOYhMAEuY4CB9NDYXS4OYwRAIRATKQetVASCRRCoZT4YjkVI0Q8DpMcXV8fNaSSWjMKbCaXSGUyWWzkCYFBBzCAFGBkByEsgeXzKXTWAg4FhOCgAAak-3IIZC5DDFB9f0CKDCC6MFjQf2S5IAoGggzgyFGuEIpGOgBeaoxoxe2PceOQBL1zTJVZzJvpqUZzNUrO5GE4CitNs4IAA5I7nSg3TnkDBYeGhihOHBqCgoBAhYiU0A

[jespertheend]

Ah sorry, I wasn't aware this was intentional. Thanks for example!