microsoft / microsoft/TypeScript
generic function parameter infers boolean as true
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Description
TypeScript Version: 3.3.0-dev.201xxxxx
Search Terms: true boolean generic function extends default
Code
const fn = <T extends boolean>(flag: T | false = false) => flag
fn({} as boolean)
// The only workaround?
fn<boolean>({} as boolean)
Expected behavior:
Expected fn<boolean>(flag?: boolean) to be inferred
Actual behavior:
Instead got fn<true>(flag?: boolean) as the inferred type.
The issue stems from T | false, which is required for = false to work.
Playground Link: Click here
Related Issues: Possibly https://github.com/Microsoft/TypeScript/issues/28154
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by reproducing the generic inference example from the issue in the linked TypeScript Playground, then compare it with the related issue #28154. Investigate the type-checker behavior for T | false and default parameters. Done means fn({} as boolean) infers boolean rather than true while preserving the default false behavior.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- typescript
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 45/100