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fix: dereference alias types in discriminated unions (#1750) #2420

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fix: dereference alias types in discriminated unions (#1750) #2420

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4c324da
fix: dereference alias types in discriminated unions (#1750)
CristianLopezR Dec 3, 2025
00d7b8d
chore: add index.test.ts as requested
CristianLopezR Dec 3, 2025
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6 changes: 5 additions & 1 deletion src/TypeFormatter/UnionTypeFormatter.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -40,7 +40,11 @@ export class UnionTypeFormatter implements SubTypeFormatter {
const kindTypes = type
.getTypes()
.filter((item) => !(derefType(item) instanceof NeverType))
.map((item) => getTypeByKey(item, new LiteralType(discriminator)));
.map((item) => {
const propertyType = getTypeByKey(item, new LiteralType(discriminator));

return propertyType ? derefType(propertyType) : undefined;
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});

const undefinedIndex = kindTypes.findIndex((item) => item === undefined);

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4 changes: 4 additions & 0 deletions test/valid-data/dereference-alias-types-discriminated-unions/index.test.ts
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@@ -0,0 +1,4 @@
import { assertValidSchema } from "../../utils";
import { test } from "node:test";

test("dereference-alias-types-discriminated-unions", assertValidSchema("dereference-alias-types-discriminated-unions", "MyUnion"));
14 changes: 14 additions & 0 deletions test/valid-data/dereference-alias-types-discriminated-unions/main.ts
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Original file line number Diff line number Diff line change
@@ -0,0 +1,14 @@
type KindA = "kind_a";
type KindB = "kind_b";

@arthurfiorette arthurfiorette Feb 10, 2026

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what happens if you export KindB? will it keep being a reference instead of inlining like

{
                            "const": "kind_a",
                            "type": "string"
                        },

?


interface ObjectA {
kind: KindA;
value: number;
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}

interface ObjectB {
kind: KindB;
name: string;
}

export type MyUnion = ObjectA | ObjectB;
Comment on lines +13 to +14

Copilot AI Feb 10, 2026

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This fixture is named/used as a discriminated-union regression test, but MyUnion is not annotated with @discriminator kind, so UnionTypeFormatter.getJsonSchemaDiscriminatorDefinition() (and the new deref logic) will never run. Add a JSDoc @discriminator kind on MyUnion to actually exercise the discriminated-union code path.

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@arthurfiorette arthurfiorette Feb 10, 2026

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This is right, the PR attempts to fix discriminated unions but your union is not discriminated

44 changes: 44 additions & 0 deletions test/valid-data/dereference-alias-types-discriminated-unions/schema.json
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{
"$ref": "#/definitions/MyUnion",
"$schema": "http://json-schema.org/draft-07/schema#",
"definitions": {
"MyUnion": {
"anyOf": [
{
Comment on lines +5 to +7

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If this is meant to validate discriminated-union output, the expected schema should reflect the @discriminator handling (i.e., a definitions.MyUnion object with allOf containing if/then branches and a discriminator properties.kind.enum). The current expected schema is a plain anyOf of the variants, which does not exercise getJsonSchemaDiscriminatorDefinition() or the new dereferencing behavior.

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"additionalProperties": false,
"properties": {
"kind": {
"const": "kind_a",
"type": "string"
},
"value": {
"type": "number"
}
},
"required": [
"kind",
"value"
],
"type": "object"
},
{
"additionalProperties": false,
"properties": {
"kind": {
"const": "kind_b",
"type": "string"
},
"name": {
"type": "string"
}
},
"required": [
"kind",
"name"
],
"type": "object"
}
]
}
}
}
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