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

[CristianLopezR]

This PR fixes issue #1750.

Previously, getJsonSchemaDiscriminatorDefinition failed to identify literal types when they were wrapped in a Type Alias (e.g., type MyKind = "kind_a").

This change adds a derefType() step when retrieving the discriminator property type in UnionTypeFormatter.ts, ensuring the underlying literal value is correctly extracted.

Fixes #1750

[Copilot]

Pull request overview

Fixes discriminated-union discriminator extraction when the discriminator property type is wrapped (e.g., via exported type aliases), by dereferencing the discriminator property type before generating if/then branches.

Changes:

• Dereference discriminator property types in UnionTypeFormatter.getJsonSchemaDiscriminatorDefinition() to correctly extract underlying literal values.
• Add a new valid-data fixture and test case intended to cover alias-type dereferencing for discriminated unions.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
src/TypeFormatter/UnionTypeFormatter.ts	Dereferences discriminator property type before definition generation.
test/valid-data/dereference-alias-types-discriminated-unions/main.ts	Adds a TypeScript fixture with alias-based discriminator property types.
test/valid-data/dereference-alias-types-discriminated-unions/schema.json	Adds the expected schema output for the new fixture.
test/valid-data/dereference-alias-types-discriminated-unions/index.test.ts	Registers the new valid-data schema assertion test.

Comments suppressed due to low confidence (1)

src/TypeFormatter/UnionTypeFormatter.ts:55

• kindTypes is built from a filtered type.getTypes() list (excluding types that deref to NeverType), but the error message uses type.getTypes()[undefinedIndex] from the unfiltered list. If any union member dereferences to NeverType, the index can point at the wrong member type in the error. Consider keeping a const filteredTypes = ... array and using it consistently for both kindTypes and error reporting.

        const kindTypes = type
            .getTypes()
            .filter((item) => !(derefType(item) instanceof NeverType))
            .map((item) => {
                const propertyType = getTypeByKey(item, new LiteralType(discriminator));

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

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

        if (undefinedIndex !== -1) {
            throw new JsonTypeError(
                `Cannot find discriminator keyword "${discriminator}" in type ${type.getTypes()[undefinedIndex].getName()}.`,
                type,
            );

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[Copilot]

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.

[arthurfiorette]

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

[Copilot]

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.

[arthurfiorette]

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

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

?