Template Literal with infer does not handle surrogate pairs

[AnyhowStep]

TypeScript Version: 4.1.0-dev.20201015

Search Terms:
template literal, surrogate pair, infer

Code

type Head<StrT extends string> = StrT extends `${infer HeadT}${string}` ? HeadT : never;

type Tail<StrT extends string> = StrT extends `${string}${infer TailT}` ? TailT : never;

type TestHead = Head<"𡁻">
type TestTail = Tail<"𡁻">

Expected behavior:
Should handle surrogate pairs properly

Actual behavior:

Playground Link:
Playground

Related Issues:
Searched for "template literal surrogate pair" and didn't find anything

[RyanCavanaugh]

We're doing what JS does here for the most part. We can revisit if there's a ton of feedback but this is just very difficult to handle and we don't expect to see these kinds of strings in practice.

> "𡁻".length
2
> "𡁻"[0]
"�"
> "𡁻"[1]
"�"
> "𡁻".match(/(.)(.)/)
(3) ["𡁻", "�", "�", index: 0, input: "𡁻", groups: undefined]
0: "𡁻"
1: "�"
2: "�"
groups: undefined