Incorrect return type from function union

[kesupile]

Bug Report

🔎 Search Terms

Incorrect union of function return type, function incorrect union

🕗 Version & Regression Information

v4.8.4 - This is the behaviour in every version I tried, and I reviewed the FAQ for entries about functions and unions

⏯ Playground Link

Playground link with relevant code

💻 Code

🙁 Actual behavior

output is { w: number; x: string; }

🙂 Expected behavior

output is { w: number; x: string; } | { w: number; x: string; y: boolean; z: symbol}

[fatcerberus]

This is due to subtype reduction: bar’s return type is a subtype of foo’s, so it’s eliminated from the union. This is by design, e.g. see #49710

[fatcerberus]

Closest thing to a canonical duplicate is probably #46449

[kesupile]

Thanks for the quick response. Is there a document I can read that explains the reasoning for the design decision; I'm curious.

[fatcerberus]

No official documentation I could find, but here’s the original PR that implemented it: #4537

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.