microsoft / microsoft/TypeScript

Bug: Circular references not allowed for template literal types

Open
#44,792 16 comments 17 reactions 0 assignees View on GitHub

Nobody has claimed this yet.

Awaiting More Feedback Suggestion
Dominant language
Go
Stars
111k
Forks
14.4k
Avg merge
1d 19h
Merged PRs (30d)
117

Description

Bug Report

🔎 Search Terms
  • circularly, template literal

Related issues: #43335.

🕗 Version & Regression Information
  • This changed between versions 4.0.5 and 4.1.5 (different bug before)
⏯ Playground Link

Playground link with relevant code

💻 Code
// you can have circular references in tuple types
type OneOrMoreArr0<C> = C | [C, ...OneOrMoreArr0<C>[]];
// note that
//
// type OneOrMoreArr0<C> = C | [C, OneOrMoreArr0<C>];
//
// is also valid
type OneOrMoreArr<C> = Exclude<OneOrMoreArr0<C>, C>;

const noEmptyArr: [] extends OneOrMoreArr<'*'> ? false : true = true;

const x1: OneOrMoreArr<'*'> = ['*'];
const x2: OneOrMoreArr<'*'> = ['*', '*'];
const x3: OneOrMoreArr<'*'> = ['*', '*', '*']

// but when you try and do the same thing with template literal types, you get an error
//
// "Type alias 'OneOrMoreStr0' circularly references itself."
//
// "Type 'OneOrMoreStr0' is not generic."
type OneOrMoreStr0<C extends string> = C | `${C}${OneOrMoreStr0<C>}`

// we should be able to do the above, and then have e.g.,
//
// const y1: OneOrMoreStr<'*'> = '*';
// const y2: OneOrMoreStr<'*'> = '**';
// const y3: OneOrMoreStr<'*'> = '***';


// if 'C' is the empty string, the above just reduces to:
//
// OneOrMoreStr0<''> = '';
//
// otherwise the same logic as with `OneOrMoreArr0` should apply
🙁 Actual behavior

You get the error:

    Type alias 'OneOrMoreStr0' circularly references itself.
    Type 'OneOrMoreStr0' is not generic.
🙂 Expected behavior

This should type check like with the tuple example - it's the same principle, just with template literals.

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start with the linked TypeScript Playground and the reduced recursive tuple and template-literal examples in the issue. Compare how recursive tuple aliases are accepted with how the template literal alias is rejected; done means the OneOrMoreStr examples type-check without the circular-reference errors.

Written by the indexing model from the issue text.

Assessment

Tech stack
typescript
Domain
compilers
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
38/100

Get new issues in your inbox

A short digest of beginner-friendly GitHub issues.