microsoft / microsoft/TypeScript

Component not always a valid JSX component depending on a generic type

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Domain: JSX/TSX Help Wanted Possible Improvement
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Description

Bug Report

🔎 Search Terms

jsx react generic valid JSX

🕗 Version & Regression Information
  • This changed between versions 5.0.4 and 5.1.3
⏯ Playground Link

Playground Link

💻 Code
import * as React from "react";

const A = <T extends unknown>({ a }: { a: T }) => (typeof a === "string" ? null : new Error("a must be a string")) as T extends string ? null : Error;

const jsxWithError = <A<string> a="a" />;

const B = A<string>;

const jsxWithoutError = <B a="a" />;

🙁 Actual behavior

Error on A in jsxWithError:

'A' cannot be used as a JSX component.
  Its type '<T extends unknown>({ a }: { a: T; }) => T extends string ? null : Error' is not a valid JSX element type.
    Type '<T extends unknown>({ a }: { a: T; }) => T extends string ? null : Error' is not assignable to type '(props: any, deprecatedLegacyContext?: any) => ReactNode'.
      Type 'Error' is not assignable to type 'ReactNode'.(2786)
🙂 Expected behavior

No error with the generic type provided (as is the case for jsxWithoutError), since we are sure we are using a valid JSX component.

Contributor guide

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First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Reproduce the linked Playground example in TypeScript 5.0.4 and 5.1.3, comparing the generic JSX use of A with the specialized B. Trace diagnostic 2786 and ensure the generic form with an explicit type argument is accepted while the existing error case remains meaningful.

Written by the indexing model from the issue text.

Assessment

Tech stack
react, typescript
Domain
compilers
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
45/100

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