Microsoft/TypeScript

`typeChecker.getTypeArguments` returns unexpected extra type argument

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#38,391 opened on May 7, 2020

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BugDomain: APIHelp Wanted

Description

TypeScript Version: 3.8.3

Search Terms: getTypeArguments

Code

Given input like

class X extends Y<string> {}

Call typeChecker.getTypeArguments(typeChecker.getTypeAtLocation(...)) on the Y<string> node.

Expected behavior: You get back one type argument (string).

Actual behavior: You get back two, string and X.

I imagine this is maybe something around how TS handles this typing internally, but as a user of the API I'm not sure I can predict when this function returns these or not.

Playground Link:

Related Issues: https://github.com/microsoft/TypeScript/issues/33693 suggests this API changed recently

Contributor guide

Tech stack
typescript
Domain
apitooling
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
3-5 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.
mostly clear
Prerequisites
TypeScript type systemType checker API basics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
15
Research direction
Investigate the `getTypeArguments` implementation in TypeScript's checker.ts. The bug is likely in how type arguments are computed for class heritage expressions, possibly related to 'this' type handling. Refer to the related issue #33693 which may indicate a recent change. Check if similar behavior occurs for other constructs.
`typeChecker.getTypeArguments` returns unexpected extra type argument · Microsoft/TypeScript#38391 | Good First Issue