Uncapitalize seems to not working within a type helper

[bedis-elacheche]

Bug Report

🔎 Search Terms

Uncapitalize

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Uncapitalize

⏯ Playground Link

Playground link with relevant code

💻 Code

type CamelCase1<
  S extends string,
  L extends string = Lowercase<S>,
  Res extends string = ""
> = L extends ""
  ? Uncapitalize<Res>
  : L extends `${infer H}_${infer T}`
  ? CamelCase1<never, T, `${Res}${Capitalize<H>}`>
  : CamelCase1<never, "", `${Res}${Capitalize<L>}`>;

type test1 = CamelCase2<"FOOBAR">
//   ^? type test1 = "Foobar"

type CamelCase2<
  S extends string,
  L extends string = Lowercase<S>,
  Res extends string = ""
> = L extends ""
  ? Res
  : L extends `${infer H}_${infer T}`
  ? CamelCase2<never, T, `${Res}${Capitalize<H>}`>
  : CamelCase2<never, "", `${Res}${Capitalize<L>}`>;

type test2 = Uncapitalize<CamelCase2<"FOOBAR">>
//   ^? type test2 = "foobar"

🙁 Actual behavior

First letter is uppercase

🙂 Expected behavior

First letter should be lowercase

[RyanCavanaugh]

Something weird is happening. Here's a more obviously-wrong computation:

type CamelCase1<S extends string, L extends string = Lowercase<S>, Res extends string = ""> =
  L extends "" ? `start_${Uncapitalize<Res>}_end` : CamelCase1<never, "", `${Res}${Capitalize<L>}`>;

type test1 = CamelCase1<"ABC">
//   ^?
// start_Abc_end

[Andarist]

When we hover over @RyanCavanaugh's CamelCase1 then we can see this definition:

type CamelCase1<
  S extends string,
  L extends string = Lowercase<S>,
  Res extends string = ""
> = L extends ""
  ? `start_${Uncapitalize<Res>}_end`
  : `start_${Uncapitalize<Res>}${Capitalize<L>}_end`;

As we can see - the falseType here looks quite different from the original. It has been simplified~ (I'm not sure if you call this process that).

The likely problem is that this conditional type is not fully deferred~. And if we consider this simplification, the observed result matches its "definition".

A similar thing happens with the original CamelCase1 by @bedis-elacheche