[Bug] Invalid union type for subject when subjects have different actions

[douglasg14b]

Describe the bug

The union type for Subject appears to be incorrect when different subjcts have different actions.

To Reproduce

type Abilities = ["create" | "manage", "campaign"] | ["create" | "delete", "user:invite"]

const ability = createMongoAbility<Abilities>();

// Intellisense suggests this, but TS actually errors indicating this is incorrect (which it is)
ability.can('delete', 'campaign')

// This is correct
ability.can('delete', 'user:invite')

Expected behavior

The union type for subject should be impacted by the action, and not suggest subjects that are not valid for a given action.

CASL Version

@casl/ability: 6.7.1

[stalniy]

I’m thinking whether it’s actually an issue with types.

Because if TS errors, then types are correct. And it seems as issue relates to IDE or typescript service inside IDE

[stalniy]

VSCode correctly highlights incorrect usage. Though it gives invalid suggestion

[stalniy]

This is how I solved it in casl:

as you can see type O is a union of possible arguments combinations but even when I try to create an array for this union of tuples, IDE provides wrong suggestions:

[stalniy]

In many places to not repeat the same logic, I have this CanParameters<T> helper type which allows me to write method declaration as function test(...args: CanParameters<TAbility>): ... and this doesn't work with function generic types

But indeed this can be improved. For example by creating an object map action => subject type + generic parameter on method:

type ActionToSubjectTypeMap<T extends [string, unknown]> = {
    [K in T[0]]: T extends any
        ? K extends T[0]
            ? T[1]
            : never
        : never
}

type PossibleAbilities =
    | ['read', 'User']
    | ['create' | 'delete', 'BlogPost']

type Mapping = ActionToSubjectTypeMap<PossibleAbilities>

But I consider this as an improvement not a bug because TS fails to compile with the current behavior