difference in typecheck with 5.8 (intersection of types breaks some compatibility)

[dartess]

Same errors, locations, and messages as TS5.8

yet another little difference (types are mostly copied from remeda)

type IterableContainer<T = unknown> = ReadonlyArray<T> | readonly [];
type Mapped<T extends IterableContainer, K> = {
  -readonly [P in keyof T]: K;
};
declare function pipe<A, B>(value: A, op1: (input: A) => B): B;
declare function map<T extends IterableContainer, U>(
  callbackfn: (value: T[number], index: number, data: T) => U,
): (data: T) => Mapped<T, U>;

type Point = { x?: number } & { y?: number };

export function update(modify?: (point: Point) => Point) {
  return pipe(
    [{ x: 0 }],
    modify
      ? map(modify)
      : map((point) => point),
  );
}

error TS2345: Argument of type '((data: { x: number; }[]) => { x: number; }[]) | ((data: { x: number; }[]) => Point[])' is not assignable to parameter of type '(input: { x: number; }[]) => { x: number; }[]'.
  Type '(data: { x: number; }[]) => Point[]' is not assignable to type '(input: { x: number; }[]) => { x: number; }[]'.
    Type 'Point[]' is not assignable to type '{ x: number; }[]'.
      Type 'Point' is not assignable to type '{ x: number; }'.
        Types of property 'x' are incompatible.
          Type 'number | undefined' is not assignable to type 'number'.
            Type 'undefined' is not assignable to type 'number'.

16     modify
       ~~~~~~
17       ? map(modify)
   ~~~~~~~~~~~~~~~~~~~
18       : map((point) => point),
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

TS 5.8 is OK with that: playground

If we merge { x?: number } & { y?: number } as { x?: number; y?: number } the error disappears (but in my core I merge different types).

[ahejlsberg]

This turns out to be the same issue as in #798. In the call to pipe we infer two candidates for B: { x: number }[] and Point[]. Neither is a supertype of the other, so inference picks the "first" candidate. With tsc this happens to be Point[], but due to differences in type ordering, with tsgo it is { x: number }[]. Unfortunately, the only inference that actually works out is Point[], so an error results with tsgo. It's entirely possible that there is some reordering of declarations that would also cause the error to occur with tsc.

The issue is easy to work around by adding a type assertion to the first argument, as in [{ x: 0 }] as Point[]. Again, ideally our inference algorithm would do better here, but there's no immediately apparent fix. There is some interesting discussion on the topic in this issue: https://github.com/microsoft/TypeScript/issues/40665.