Microsoft/TypeScript

Improve diagnostic when string from property is not assignable to string literal.

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#26,413 opened on Aug 13, 2018

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 (3 comments) (0 reactions) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
Domain: Error MessagesEffort: ModerateHelp WantedSuggestion

Description

TypeScript Version: 3.1.0-dev.20180810

Code

interface IceCreamOptions {
    readonly cherry: "yes" | "no"
}
declare function iceCream(options: IceCreamOptions): void;
const options = { cherry: "yes" };
iceCream(options);

Expected behavior:

Error message recommending to add a type annotation to options. This should be detectable because the source of the failing property is a PropertyAssignment in an object literal being assigned to a variable. (If there is no variable assignment, we could still recommend using ``"yes" as "yes"although that isn't as pretty.) Could also come with a codefix. Note that in JS the fix is slightly different: put/** @type {IceCreamOptions} /before theconstor use/* @type {"yes"} */ ("yes")`.

Actual behavior:

Error messages says Type 'string' is not assignable to type '"yes" | "no"'. with no information on how to fix it.

Contributor guide

Tech stack
typescript
Domain
developer experience
Issue type
feature
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.
1-2 days
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 string literal typesTypeScript type checkingObject literal property inference
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
55
Research direction
Investigate the TypeScript compiler source code, particularly the type checker (checker.ts), to understand how assignability errors are reported. Look at existing error messages for type compatibility and explore how to detect when the source is a property assignment in an object literal. Review any linked discussions or PRs (none visible) for additional context.
Improve diagnostic when string from property is not assignable to string literal. · Microsoft/TypeScript#26413 | Good First Issue