Microsoft/TypeScript

Regression: incorrect return type when disabled strictNullChecks

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#60,187 opened on Oct 10, 2024

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 (0 comments) (0 reactions) (0 assignees)TypeScript (6,726 forks)batch import
Domain: check: Type InferenceHelp WantedPossible Improvement

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Description

🔎 Search Terms

"ts7011", "strictnullchecks"

🕗 Version & Regression Information

  • This changed between versions 5.6.3 and 5.7.0-beta (also in 5.7.0-dev.20241010)

When disabled strictNullChecks in tsconfig.json error TS7011: Function expression, which lacks return-type annotation, implicitly has an T return type

⏯ Playground Link

https://www.typescriptlang.org/play/?strict=false&strictNullChecks=false&alwaysStrict=false&noEmitHelpers=true&noUnusedLocals=true&esModuleInterop=false&declaration=false&importHelpers=true&experimentalDecorators=true&target=6&module=6&suppressImplicitAnyIndexErrors=true&strictBuiltinIteratorReturn=true&ts=5.7.0-dev.20241010#code/CYUwxgNghgTiAEAzArgOzAFwJYHtXwHcsJgxZgAVHAJRAHMBRADwAcAKAZwC54OMYsqOgEoetRq3gAfeKmQQIAbgBQYPH3gx5IDvAC88ANoByKMYA08YwCNjAXQB0iaBgCyUdlogI9APkLEpORU4sye2sLSMoZ2wopAA

💻 Code

declare function wildcardToRegExp(s: string): RegExp | null;
const rules = ['a', 'b'].flatMap(rule => wildcardToRegExp(rule) || []); // if flatMap<RegExp> - no error

🙁 Actual behavior

TS7011: Function expression, which lacks return-type annotation, implicitly has an 'RegExp | any[]' return type.

🙂 Expected behavior

No error

Additional information about the issue

No response

Contributor guide

Research direction
Investigate the change in TypeScript versions 5.6.3 to 5.7.0 beta that causes regression when strictNullChecks is disabled. Focus on the inference of return types in callbacks passed to flatMap, especially how union types with array literals are handled.
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.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
TypeScript
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60