Microsoft/TypeScript

Services for non-homomorphic mapped type

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#25,607 opened on Jul 12, 2018

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Domain: LS: Symbol NavigationDomain: Mapped TypesExperience EnhancementHelp WantedSuggestion

Description

TypeScript Version: 3.0.0-dev.20180712

Code

interface I {
 a: number;
 b: number;
}
type J = { [K in "b"]: I[K] };
declare const j: J;
j.b; // Go-to-definition does not work

Expected behavior:

Since the type we're getting is I[K], the mapped property symbol should have a reference back to the corresponding property in I.

Actual behavior:

The mapped type creates an unrelated property symbol and services don't work.

Contributor guide

Tech stack
typescript
Domain
developer experiencetooling
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.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
TypeScript mapped typesTypeScript compiler internalsSymbol resolution
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
25
Research direction
Investigate how the TypeScript compiler creates property symbols for mapped types, particularly in the checker.ts file. Focus on the handling of non homomorphic mapped types (e.g., `type J = { [K in "b"]: I[K] }`) where the property symbol should reference the original property in the base type. Review the code that builds mapped type symbols and ensure that the go to definition service correctly resolves to the source property.
Services for non-homomorphic mapped type · Microsoft/TypeScript#25607 | Good First Issue