Characters count as both prefix and suffix of pattern literal

[andrewbranch]

TypeScript Version: v4.1.0-dev20200925

Search Terms: pattern literal prefix suffix

Code

const s: `:${string}:` = ":";

Expected behavior:
Assignability error because the pattern `:${string}:` has two literal : characters, while ":" has only one.

Actual behavior:
No error

Playground Link

Related Issues: #40598

[jamesmfriedman]

Related to this issue,

• It's great that in dev you can use string as a value placeholder! This is a marked improvement from beta.
• The ability to define a prefix or suffix for a key is very valuable, but currently neither of these usecases work:

type CustomProps = Record<`x${string}`, any>;
const a: CustomProps = {};
a.x11 = ''; // ERRORS OUT

type CustomProps ={[key: `x${string}`]: any}; // ERRORS, key must be string or number
const a: CustomProps = {};

Would be amazing if this could work!