Why can't we use the toLowerCase() and toUpperCase() string methods to solve the Lowercase and Uppercase utilities?

[YadBro]

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• This is a crash

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interface ITodo {
    activity: Lowercase<string>, // (Error) Type 'string' is not assignable to type 'Lowercase<string>'.
    description: Uppercase<string>, // (Error) Type 'string' is not assignable to type 'Uppercase<string>'.
}

const todo: ITodo = {
    activity: 'Fishing'.toLowerCase(),
    description: 'Fishing with your mom'.toUpperCase(),
}
console.log(todo);

[MartinJohns]

Because both toLowerCase() and toUpperCase() return a string, not Lowercase<string> or Uppercase<string>. Changing this would be a breaking change. The issue for this is still open: #44268

[YadBro]

I'm still confused, I think it should work. Well don't think about it. Thanks for the explanation. I will close this issue thank you.

[fatcerberus]

I'm still confused, I think it should work.

.toLowerCase() is just another method that returns a string, like any other; the compiler doesn’t have any context to know this particular string is compatible with Lowercase<string>.