Variadic with "named tuples" doesn't preserve name

[ChuckJonas]

Not sure this actually should be possible... I guess it would get funky as typescript would have to deal with naming conflicts.

TypeScript Version: 4.1.0-dev

Search Terms:
Variadic, Named Tuples
Code

type Append<I, T extends unknown[]> = [...T, last:I];
type FooBar = Append<string, [foo: number, bar: boolean]>

Expected behavior:
Should preserve named tuples labels:

type FooBar = [foo: number, bar: boolean, last: string]

Actual behavior:

type FooBar = [number, boolean, string]

Playground Link:
https://www.typescriptlang.org/play?ts=4.0.0-beta#code/C4TwDgpgBAgmkDsAmAeAkgGigFShAHsBMgM5QCuCA1ggPYDuCA2gLoB8UAvFEwHT-YsAGwCGJYAC40LANwAoUJCgAxWrQBCIgE5dY8YqnFaAlggDmWJgDM1EqAnIBbAEYQtWZ9rvO1QiCIR2OSA

Related Issues:
none

[DanielRosenwasser]

@ahejlsberg any idea how feasible this would be? One problem is you don't know whether two spread instantiations will end up with identical names.

[xuld]

In most use case, names should be unique.

My proposal:

[...[foo: number, bar: boolean], foo: string] // --> [foo: number, bar: boolean, foo_2: string]
[...[...[foo: number, bar: boolean], foo: string], foo: string] // --> [foo: number, bar: boolean, foo_2: string, foo_3: string]
[foo: string, ...[foo: number, bar: boolean]] // --> [foo: string, foo_2: number, bar: boolean]

[0az]

Setting aside the generic case, having name preservation for simpler cases would be great:

type Foo = [foo: string]
type FooBar = [...Foo, bar: string]
// src/index.ts:2:22 - error TS5084: Tuple members must all have names or all not have names.

https://www.typescriptlang.org/play?ts=4.0.2#code/C4TwDgpgBAYg9nKBeKBtAZggXFAzsAJwEsA7AcwF0AoUSWBAIQEMDk0A6T+OAGigCMWOfMXLUqAYzgl8UTHEGsUqAOTyVfFYpXUpMuABsI7A3DIAKAAYASAN7zFAX0sBKIA

This doesn't error on playground because of #40118, which landed after 4.0.2.

[szymski]

I hit the same problem. I found an ugly workaround which looks like this:
type Append<A, B> = A extends [...infer Params] ? [...Params, ...(B extends [...infer Params2] ? Params2 : [])] : never;
Labels are preserved.

[ahejlsberg]

We already support giving names to variadic elements, and the original example works as expected when you do so:

type Append<I, T extends unknown[]> = [...first: T, last: I];
type FooBar = Append<string, [foo: number, bar: boolean]>;  // [foo: number, bar: boolean, last: string]
type FooBaz = Append<string, number[]>;  // [...first: number[], last: string]

Note that when a variadic element is instantiated with a tuple type, the individual element names of that tuple type override the name of the variadic element.

All in all, I don't think any new features or changes are needed here.