Microsoft/TypeScript

Type inference regression on function calls not saved to variables, due to #61668

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#62,071 opened on Jul 15, 2025

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Domain: check: Type InferenceHelp WantedPossible Improvement

Description

🔎 Search Terms

61668 inference overload regression tuple

🕗 Version & Regression Information

  • This changed in commit or PR #61668

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.0-beta#code/PTAEGIDYEZMgOUBjAFgQwHYHMCmBnUAFxR1AEsMAzHAJxwyVMpoHsBbUAbQwFc2AjWgBpQeQjQpYAukRagAFLwG0APmInYAlJykAoACY4kAGzR1QlHg0JkWGULkIA5PvM0AuUAAVWbMnhwAHiVBGgA+AG4DI1NzS2tbe0cAZXE3Tx92fyD1SUjokzMmKyQbO1AeAIAVHgAHYxx5QjqGz24+UJFc7CkPUAA3FjJ9KKQ7MVBa0ABeb19sgDo6PBZjfsadTQXienk3GbC5rICFtGNjeU5HFzY3ERS0zV6tnYx5Spwa+pxNKKA

💻 Code

declare function getNum(): Promise<number>;
declare function getStr(): Promise<string>;
declare function useTuple(tuple: [number, string]): void;
const p = Promise.resolve([]).then(() => Promise.all([getNum(), getStr()])).then(useTuple);

🙁 Actual behavior

The Promise.all infers to Promise<(number|string)[]>.

🙂 Expected behavior

The Promise.all should infer Promise<[number, string]>.

Additional information about the issue

This only happens when the Promise.all call is directly returned. If I instead write

const p = Promise.all([getNum(), getStr()]);
return p;

then it infers the tuple correctly again, even without doing anything that would suggest it should infer the type differently.

This PR also impacted inference of enums: in another example, I pass {mode: new Subject(Mode.A)} into a function that fails because it infers Subject<Mode.A> while if I save a temporary const mode = new Subject(Mode.A); then it infers Subject<Mode> and passes.

Contributor guide

Tech stack
typescript
Domain
developer experiencetooling
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
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 basicsunderstanding of generics and inference
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
65
Research direction
Investigate the change introduced in PR #61668, which likely modified the type inference logic for function calls. The issue shows that when the result of Promise.all is used directly in a .then() call, tuple inference fails, but storing it in a variable works. Look into the TypeScript checker code that handles inference of generic function calls and contextual types, specifically the part that differs when the call is not saved to a variable. Check if the regression is due to how contextual types are propagated in that scenario.