Merging tuples in templates looses labeling

[brett19]

Bug Report

🔎 Search Terms

• labeled tuple
• tuple merging

🕗 Version & Regression Information

This is a bug in all versions of Typescript I tested which support labeled tuples, variadic templates and non-last rest parameters in tuples (4.2.x)

⏯ Playground Link

Playground link with relevant code

💻 Code

// Some helpers
type DropFirst<T extends any[]> = T extends [infer _, ...infer T] ? T : never
type Last<T extends any[]> = T extends [...infer _, infer L] ? L : never

// Some setup
type TestFunc = (a: string, b: (x: number, y: string) => void) => void
type TestParams = Parameters<TestFunc>

// Works without a template.  Yields [z: Error, y: string]
type TestTypeWorks =
  [...[z: Error], ...DropFirst<Parameters<Last<TestParams>>>]

// Breaks when used with a template?  Yields [Error, string]
type TestTypeTpl<T extends [...any[], (...args: any) => any]> =
  [...[z: Error], ...DropFirst<Parameters<Last<T>>>]
type TestTypeFails = TestTypeTpl<TestParams>

// Individual pieces work fine
// Yields [y: string]
type TestTypeTpl2<T extends [...any[], (...args: any) => any]> =
  [...DropFirst<Parameters<Last<T>>>]
type TestTypeWorksX = TestTypeTpl2<TestParams>

// Yields [z: Error]
type TestTypeTpl3<_ extends [...any[], (...args: any) => any]> =
  [...[z: Error]]
type TestTypeWorksY = TestTypeTpl3<TestParams>

// But then they fail again when we put them together. Yields [Error, string]
type TestTypeFailsTpl4<T extends [...any[], (...args: any) => any]> =
  [...TestTypeTpl2<T>, ...TestTypeTpl3<T>]
type TestTypeWorksZ = TestTypeFailsTpl4<TestParams>

🙁 Actual behavior

The tuples lost their labels.

🙂 Expected behavior

The tuples should still have their labels.

[brett19]

This might be a dupe of #42106 or #39941. There is an implementation of Append which definitely does work around this issue there though, but I can't seem to figure out what's special about my instance that makes the names fail to be propagated.

type Append<A, B> = A extends [...infer Params] ? [...Params, ...(B extends [...infer Params2] ? Params2 : [])] : never;

Edit: In hindsight this only half-solved the issue. If I'm trying to adjust the parameter list tuple of a node-callback style function (from the end of my input function), it again looses the names, even using the Append implementation above.

Here is a more concise example of that issue:

type SomeTuple = [callback: (x: number, y: boolean) => void]

type InferNestedArgs<T> = T extends [(...cbArgs: infer CbArgs) => void]
  ? CbArgs
  : never

type Works = [...[added: Error], ...InferNestedArgs<SomeTuple>]

type BrokenTpl<T> = [...[newErr: Error], ...InferNestedArgs<T>]
type Fails = BrokenTpl<SomeTuple>

type MakeError<T extends never> = T
let a: MakeError<Works> // yields [added: Error, x: number, y: boolean]
let b: MakeError<Fails> // yields [Error, number, boolean]

[RyanCavanaugh]

Preserving tuple labels through transforms is "best effort" and not guaranteed to work in call cases. If it's an easy fix we'd take a PR but don't want to do anything perf-impacting for a nice-to-have.

[DanielRosenwasser]

This is fixed in TypeScript 5.2, probably by #53356 (see the blog post).