[API] ConditionalType has no counterpart to root of TypeScript 6 (the node it was declared with) #64740

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Comment

In TypeScript 6 a conditional type has root: ConditionalRoot (public in typescript.d.ts), and root.node is the ConditionalTypeNode it was declared with. In the TypeScript 7 API, ConditionalType has getCheckType(), getExtendsType(), getTrueType() and getFalseType(), and nothing that leads to that node. For an instantiated conditional type without an alias symbol I have found no way to get its branches as they were declared.

declare class Box<T, V> {
  get(): T extends string ? V : never;
}
export function h<U>(box: Box<U, { a: number }>) {
  return box.get();
}

For the type of box.get(), which prints as U extends string ? { a: number; } : never in both versions:

On main (aa81492), tsc/internal/api/proto.go still sends only checkType and extendsType for a conditional type, and the methods are getTrueTypeOfConditionalType and getFalseTypeOfConditionalType, so I believe this is the same in 7.1.

Use case. typescript-eslint's isTypeReadonly (behind prefer-readonly-parameter-types) looks at the declared branches:

[type.root.node.trueType, type.root.node.falseType].map(checker.getTypeFromTypeNode)

With TypeScript 6 it answers true for the type above (V and never are readonly). A port to the new API can only look at the instantiated branches, and { a: number } is not readonly, so it would answer false.

Request. A way to get the declaration node of a conditional type, for example ConditionalType.getRoot() with node and isDistributive as in TypeScript 6. #64533 asks for getInferTypeParameters(), which lived on the same root object.