Intersect with discriminated type

[Kal-Aster]

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🔎 Search Terms

"discriminated"
"intersect"
"ParseError: T must be assignable to object"

🧩 Context

• ArkType version: 2.0.0-rc.30
• TypeScript version (5.1+): 5.7.2
• Other context you think may be relevant (JS flavor, OS, etc.):

🧑‍💻 Repro

import { scope } from "arktype";

scope({
    AEditing: {
        edits: {
            a: "number"
        },
        nodes: "(Omit<NodeA, 'editableA'> | Omit<NodeB, 'editableA' | 'edits'>)[]"
    },
    NonEditing: {
        edits: "undefined?",
        nodes: "(NodeA | NodeB)[]"
    },
    NodeA: {
        type: "'a'",
        editableA: "number"
    },
    BaseNodeB: {
        type: "'b'",
        editableA: "number"
    },
    Editing: "AEditing | NonEditing",
    NodeB: "BaseNodeB & Editing",
    Node: "NodeA | NodeB"
}).export();

I get the following error:

D:\***\node_modules\@ark\util\out\errors.js:5    throw new ctor(message);
          ^

ParseError: T must be assignable to object (was $AEditing&intersection250 | { editableA: number, nodes: ($NodeB | { editableA: number, type: "a" })[], type: "b", edits?: undefined })    
    at throwError (D:\***\node_modules\@ark\util\out\errors.js:5:11)
    at throwParseError (D:\***\node_modules\@ark\util\out\errors.js:10:43)
    at <anonymous> (D:\***\node_modules\@ark\schema\out\generic.js:21:21)
    at <anonymous> (D:\***\node_modules\@ark\util\out\flatMorph.js:4:39)
    at Array.flatMap (<anonymous>)
    at flatMorph (D:\***\node_modules\@ark\util\out\flatMorph.js:3:39)
    at GenericRoot.<anonymous> (D:\***\node_modules\@ark\schema\out\generic.js:18:30)
    at parseGenericInstantiation (D:\***\node_modules\arktype\out\parser\shift\operand\unenclosed.js:18:12)    at maybeParseReference (D:\***\node_modules\arktype\out\parser\shift\operand\unenclosed.js:40:16)      
    at unenclosedToNode (D:\***\node_modules\arktype\out\parser\shift\operand\unenclosed.js:20:40)

Node.js v22.12.0

I need the validation for a type that can have this "edits" property and based on the presence of this property and its actual structure (it could be another type of edits, in the actual usage) I need to narrow the type of "nodes".

In TS it works:

type AEditing = {
    edits: {
        a: number
    },
    nodes: (Omit<NodeA, "editableA"> | Omit<NodeB, "editableA">)[]
}
type NonEditing = {
    edits?: undefined,
    nodes: (NodeA | NodeB)[]
}
type NodeA = {
    type: "a",
    editableA: number
}
type NodeB = {
    type: "b",
    editableA: number
} & (AEditing | NonEditing);
type Node = NodeA | NodeB;

const node: Node = {
    type: "b",
    editableA: 12,
    edits: {
        a: 23
    },
    nodes: [
        {
            type: "b",
            // @ts-expect-error
            editableA: 23,
            nodes: []
        }
    ]
}

I'm new to this library: is this actually a bug or am I doing something wrong?

[ssalbdivad]

This issue is similar to #1188. Unfortunately, there are still a few issues related to complex cyclic unions.

Handling these precisely will require a more significant refactor as outlined in #1026.

In the meantime, the best workaround would be to just avoid the problematic cyclic reference directly in the type and instead recurse by invoking the type directly from a .narrow.