google/closure-compiler

Cryptic "Bad Annotation" Error Message

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#2489 opened on May 12, 2017

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P3help wanted

Description

A variable assignment with 2 @extends annotations gave this warning using NTI:

WARNING - Bad type annotation. type annotation incompatible with other annotations. See https://github.com/google/closure-compiler/wiki/Bad-Type-Annotation for more information. That could be a lot better (more helpful).

/**
  * @constructor
  * @extends {Polymer.Element}
  * @extends {ObservableModelsElement}
  */
const CardElement = observableModelsMixin(cardUtilsMixin(Polymer.GestureEventListeners(Polymer.Element)));

Contributor guide

Tech stack
javascriptjava
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.
2
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
basic JavaScriptfamiliarity with Closure Compiler type annotations
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
80
Research direction
Investigate where in the Closure Compiler source code the warning 'Bad type annotation' is generated. The warning message is likely in a Java file that handles type annotation validation. Look for the string 'Bad type annotation' in the compiler source. Once located, improve the message to indicate that multiple @extends annotations are not allowed or provide more context, as shown in the example.