Microsoft/TypeScript

Combination of intersection type, mapped type and generic type seem to break type checks for nested properties

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#49,638 opened on Jun 22, 2022

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BugDomain: Mapped TypesHelp Wanted

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Description

Bug Report

🔎 Search Terms

mapped types, generic bound, nested objects

🕗 Version & Regression Information

We've detected this on 4.7.4. Checking previous versions on playgrounds, last one where it worked correctly is 3.5.1. It is also reproducible on current nightly build.

  • This changed between versions 3.5.1 and 3.6.3

⏯ Playground Link

Playground link with relevant code

💻 Code

type BaseType = {
  select?: { id?: boolean} ,

  where: {},
}

type FullType = BaseType & {
  anything?: boolean
}

type MappedType<T> = {
  [key in keyof T]:  T[key]
}

declare function genericFunction<T extends FullType>(args: MappedType<T>): void

genericFunction({
  where: {},
  select: {
    id: true,
    // this should not pass
    shouldErrorHere: 'lalalala'
  }
})

🙁 Actual behavior

The code passes typechecks

🙂 Expected behavior

shouldErrorHere property should not be allowed in that position, since it's not defined on typeof BaseType['select']. Doing any of the following causes expected error to appear:

  1. Removing intersection type and making FullType a plain alias for BaseType
  2. Making where property optional in BaseType definition
  3. Removing anything property from FullType
  4. Adding any non-optional property to the intersection
  5. Omitting where property in genericFunction call
  6. Passing incorrect anything value during the call
  7. Changing FullType to interface FullType extends BaseType

Contributor guide

Research direction
Examine the TypeScript compiler's handling of mapped types over generic types constrained to intersection types, focusing on the interaction with optional properties in the base type. Investigate why excess property checking is not triggered when the mapped type wraps a generic parameter that extends an intersection type with optional properties. Look at the type checker's logic for mapped types and generic constraints, particularly the handling of intersection types and optionality.
Tech stack
typescript
Domain
developer experience
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.
Half day
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.
30