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Relating large unions of literal types performs poorlyΒ #53191

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#53192
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Relating large unions of literal types performs poorly#53191
#53192
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ahejlsberg
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Domain: PerformanceReports of unusually slow behaviorFix AvailableA PR has been opened for this issue
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TypeScript 5.1.0

Description

@ahejlsberg
Anders Hejlsberg (ahejlsberg)
opened on Mar 10, 2023

πŸ•— Version & Regression Information

All versions.

⏯ Playground Link

Playground link with relevant code

πŸ’» Code

type Digits = '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9';
type T9XXXX = `9${Digits}${Digits}${Digits}${Digits}`;
type TXXXXX = `${Exclude<Digits, '0'>}${Digits}${Digits}${Digits}${Digits}`;

let x: Exclude<TXXXXX, T9XXXX> = '88888';

πŸ™ Actual behavior

The example above takes ~45s to compile on a reasonably fast machine. The underlying issue is that as we relate large unions of primitive types, we have a fast path for matches, but not for mismatches. Because the majority of the constituents of TXXXXX are not present in T9XXXX, we spend a lot of time in the slow path that attempts to find matches by iterating through all constituents.

πŸ™‚ Expected behavior

We can do much better by implementing a fast path for mismatches.

Activity

  1. Anders Hejlsberg (ahejlsberg)
    self-assigned this
    on Mar 10, 2023
  2. Anders Hejlsberg (ahejlsberg)
    added this to the TypeScript 5.1.0 milestone on Mar 10, 2023
  3. Ryan Cavanaugh (RyanCavanaugh)
    added
    Domain: PerformanceReports of unusually slow behavior
    on Mar 10, 2023
  4. TypeScript Bot (typescript-bot)
    added
    Fix AvailableA PR has been opened for this issue
    on Mar 10, 2023

  5. fatcerberus commented on Mar 10, 2023

    @fatcerberus
    Bruce Pascoe (fatcerberus)
    on Mar 10, 2023

    The example above takes ~45s to compile on a reasonably fast machine.

    So more out of morbid curiosity than anything...

    fatcerberus@Bruces-MacBook-Pro temp % time npx tsc wtf.ts
npx tsc wtf.ts  60.97s user 0.17s system 101% cpu 1:00.28 total
fatcerberus@Bruces-MacBook-Pro temp % node -v
v19.7.0

    That was with an M2 Pro. Holy crap.

  6. Anders Hejlsberg (ahejlsberg)
    closed this as completedin #53192on Mar 17, 2023
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