Microsoft/TypeScript

`x && x` should not be `NonNullable`

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#62,133 opened on Jul 27, 2025

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 (5 comments) (5 reactions) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
BugDomain: check: Control FlowHelp Wanted

Description

🔎 Search Terms

logical and, generic, NonNullable

🕗 Version & Regression Information

There has been this problem since 4.8.4 (I tried it on the playground)

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/MYewdgzgLgBAZjAvDAPAFQDQD4AUAPALhjQEoksY8YAyaymAegdTV0OLMQoDlxuBXADaCAhgCNBAU3RYAUEA

💻 Code

const f = <T,>(x: T) => x && x // <T>(x: T) => NonNullable<T>

🙁 Actual behavior

Since f returns NonNullable<T>, we can make every value NonNullable, and never can be created by calling f(null).

🙂 Expected behavior

f should return T.

Additional information about the issue

No response

Contributor guide

Tech stack
typescript
Domain
tooling
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.
active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
understanding of TypeScript type systemfamiliarity with control flow analysis
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
25
Research direction
Investigate the TypeScript checker's handling of logical AND expressions with generics. Specifically, look at how the type narrowing for `x && x` is implemented in src/compiler/checker.ts. The issue suggests that the result should be `T` rather than `NonNullable<T>`. Review the narrowing logic for `&&` and `||` and consider previous related issues or PRs. The playground link provides a minimal reproduction; start by understanding the expected vs actual behavior in the context of TypeScript's type system.