Microsoft/TypeScript

Incorrect inference/autocompletion on generic arrays, when values can be inferred from a defined object.

Open

#41,645 opened on Nov 23, 2020

View on GitHub
 (5 comments) (5 reactions) (0 assignees)TypeScript (6,726 forks)batch import
Domain: LS: Completion ListsEffort: ModerateExperience EnhancementExperimentation NeededHelp WantedSuggestion

Repository metrics

Stars
 (48,455 stars)
PR merge metrics
 (Avg merge 6d 17h) (9 merged PRs in 30d)

Description

TypeScript Version: 4.1.2

Search Terms: Autocompletion, incorrect, values, inference, generic, array, object, keys

Summary: When an interface/a type has an object with generic keys, and an array of those keys, the array values cannot be infered from the object keys.

Code

interface Recipe<INGREDIENTS extends string> {
  quantities: Record<INGREDIENTS, number>
  allergens?: INGREDIENTS[]
}

function createRecipe<INGREDIENTS extends string>(recipe: Recipe<INGREDIENTS>) {}

createRecipe({
  quantities: {
    eggs: 1,
    flour: 2,
  },
  allergens: ['']
})

Expected behavior: Here, when trying to give a value to allergens, the autocompletion should show "eggs" | "flour".

Actual behavior: The autocompletion doesn't find anything. image

Notes: The other way is working: you can fill the array first, then the object keys will autocomplete - but this rarely make sense to write things that way.

Failed workarounds: This bug is still present, even when:

  • allergens is optionnal
  • We switch from an interface to a type
  • We use keyof this['quantities'] instead of INGREDIENTS[]

Contributor guide

Research direction
Investigate the type inference order in generic functions: how the compiler resolves dependencies between properties in an object literal. Specifically, when a generic type parameter is inferred from one property (e.g., quantities), the other property (allergens) should use that inferred type. Look into the type checker's handling of contextual typing for array literals in the presence of generic constraints.
Tech stack
typescript
Domain
compilers
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
TypeScript
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
65