Bad inference for param of type T[] | T[][]

[jsnajdr]

Steps to reproduce

Try to compile this code:

function isNestedArray<T>(arr: T[] | T[][]): arr is T[][] {
	return Array.isArray(arr) && Array.isArray(arr[0]);
}

type TG = { a: string };

function convert(controls: TG[] | TG[][]): TG[][] {
  if (isNestedArray(controls)) {
    return controls;
  } else {
    return [ controls ];
  }
}

(for example, in the tsgo.sxzz.dev playground)

Behavior with typescript@6.0

In 6.0 the code compiles cleanly, and the T parameter of isNestedArray is correctly inferred as TG.

Behavior with tsgo

With tsgo there is an error:

main.ts(8,21): error TS2345: Argument of type 'TG[][] | TG[]' is not assignable to parameter of type 'TG[][][] | TG[][]'.
  Type 'TG[]' is not assignable to type 'TG[][][] | TG[][]'.
    Type 'TG[]' is not assignable to type 'TG[][][]'.
      Type 'TG' is missing the following properties from type 'TG[][]': length, pop, push, concat, and 35 more.
main.ts(9,5): error TS2322: Type '(TG[][] | TG[]) & TG[][][]' is not assignable to type 'TG[][]'.
  Type 'TG[] & TG[][][]' is not assignable to type 'TG[][]'.
    The types returned by 'pop()' are incompatible between these types.
      Type 'TG | undefined' is not assignable to type 'TG[] | undefined'.
        Type 'TG' is missing the following properties from type 'TG[]': length, pop, push, concat, and 35 more.
main.ts(11,14): error TS2322: Type 'TG[][] | TG[]' is not assignable to type 'TG[]'.
  Type 'TG[][]' is not assignable to type 'TG[]'.
    Property 'a' is missing in type 'TG[]' but required in type 'TG'.

Apparently the inference is "one array level off", where T is inferred as TG[] instead of TG.

A necessary condition to reproduce is that TG is a "complex" type like { a: string }. With a primitive type like string the bug doesn't happen.

[JoostK]

Sounds like #1789