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@typescript-bot
I remove that commit with force-push!
@catb0t
Can you check if it's fixed as you intended??
@DanielRosenwasser
I wanna get some feedback!

I don't speak for the typescript staff, but I still think the error is too vague.
I read "either string or number", and still think string | number, because that type can be either a string, or a number.
As suggested in #33427, I would like to see a suggestion as to what the dev is trying to do, like a Record.
I'm not sure how using a union as a key would result in a mapped type, but maybe somebody can clear that up, and we could potentially have that suggestion there too.

I think two messages mean:

either string | number -> you need to
use Record -> recommend you instead of union

How about thought of others?
Feel free to leave message :)

Thoughts @fatcerberus?

I agree with @sploders101 that the addition of "either" does next to nothing to improve clarity compared to the current message. My preference was always for the message that was suggested under Expected Behavior in #33427:

An index signature parameter type cannot be a union type. Consider using a mapped object type instead.

"Mapped object type" can be replaced with whatever is the name of the construct Record is defined in terms of. Alternatively, just suggest using Record since it's built in.

@fatcerberus
No worries :)
I didn't change only message.
I fixed the condition like this:

if (type.flags & TypeFlags.Union && allTypesAssignableToKind(type, TypeFlags.StringOrNumberLiteral, /*strict*/ true))

So we can see the error message what you want!