Union is not seen as synonymous with Generic

I'm having trouble with the code snippet below. It seems like the K generic is not being treated as a union, causing unexpected behavior. Can anyone help me figure out what's going wrong here?

type GraphSubAttributeNames<N extends ApiModelNames, K extends GraphSubAttributeKeys<N>> =
GraphSubAttributesMap<N> extends never ?
    'never' :
    GraphSubAttributesMap<N>[K] extends MapRelation ?
        GetGraphRelationMap<N, K> extends never ?
            'case 1' :
            'case 2' :
        'case 3'


type T1 = GraphSubAttributeNames<'artist', 'portrait'> // 'case 3', which is correct
type T2 = GraphSubAttributeNames<'artist', 'heroArtwork'> // 'case 2', which is also correct
type T3 = GraphSubAttributeNames<'artist', 'heroArtwork' | 'portrait'> // case 3, but should be 'case 3' | 'case 2'

Unfortunately, I haven't been able to replicate this issue with a minimal example.

Some additional types that may be relevant:

import {Artist, Footer, Site, Artwork, StrapiUser, Navigation, Exhibition, StrapiMedia} from "~/models"

// More type definitions...
typescriptgenericsunion-typesmapped-types

Answer №1

The reason for not achieving the desired outcome is due to a lack of distribution in your union check. To fix this issue, you need to individually check each item in the union, which can be accomplished in various ways. One approach is to include K extends K at the start of GraphSubAttributeNames. Think of K extends K as iterating through the union like a loop, starting with "heroArtwork" and then moving on to "portrait".

Try it out here!

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