Microsoft/TypeScript

Error message for invalid string literal type could be improved

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#10,203 opened on Aug 8, 2016

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 (1 comment) (0 reactions) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
Domain: Error MessagesEffort: ModerateExperience EnhancementHelp WantedSuggestion

Description

TypeScript Version: 2.0 Beta

Code

// A *self-contained* demonstration of the problem follows...
interface Foo {

    method(value: 'aa'): void;
    method(value: 'bb'): void;
    method(value: 'zz'): void;
    method(value: 'last'): void;
}

const x: Foo = {} as any;

x.method('bar'); // Error line

Expected behavior:

A more meaningful error message, e.g.

Argument of type "'bar'" is not assignable to parameter of type "'aa' | 'bb'|...|'last'"

Actual behavior:

Argument of type "'bar'" is not assignable to parameter of type "'last'"

I am finding this error message confusing, because the compiler picks the value of the last overload in order to report the error, and there is nothing special about the last value.

Contributor guide

Tech stack
typescript
Domain
developer experiencetooling
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.
1-3 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 type system basics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
55
Research direction
Investigate the TypeScript compiler's error reporting for overloaded function signatures with string literal types. Look for where error messages are generated for argument types, particularly in the checker.ts file. The goal is to display the union of all valid literal types instead of just the last one. Existing comments or PRs may provide direction.
Error message for invalid string literal type could be improved · Microsoft/TypeScript#10203 | Good First Issue