Contextual narrowing fails inside thunked argument when T is inferred from a separate argument

[johanrd]

🔎 Search Terms

thunk inference contextual narrowing
arrow function return contextual typing
nested array literal widening
generic inference across arguments
callback return literal narrowing
NoInfer (unrelated — same bug without it)

🕗 Version & Regression Information

• This is the behavior in every version I tried (4.9.5, 5.5.4, 5.9.3, 6.0.0-dev.20260416), and I reviewed the FAQ.

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=6.0.0-dev.20260416#code/C4TwDgpgBAwg9gGzgJygXigcmRAJpqAHywCMEBXCTAbgChRJYBDZZASwmQB4AVAPnRQA3lAD6o4AH4AXFAAUAD1k8AlOgEA3OG1xQAvnVq4IAYwQtoAM3IA7E8DZwbUXGxz3efOSdkwW7Tk8AGig4MGAAZ1kRE0QUKKgcJlwnBBAoHgBtAF19FWU6YzMLKGs7BycoYAALWwBrPE9vX38Obn4QsMjZOTU0ARi45ASklJs0jJy8gtpaWJsI4CgTQRtyBAQoJgioeps4AHdnbeZWNq54JGQ+Q1d3YG8QwauEzOw8TFy9FWooP-+AVAAPRAqCAUHIoDZ-IcEjxBJcUPRajYGrhHvI+gI5M94rI3jh8F8VD8-iCoIAZcigi3YNgA5lN9ktthE2LSoWRoMA4IkIMlUukEcgckA

💻 Code

type Color = 'red' | 'blue';
type Carrier<T> = { __t?: (x: T) => void };

declare function direct<T>(c: Carrier<T>, opts: { colors: readonly T[] }): T;
declare function thunked<T>(c: Carrier<T>, opts: () => { colors: readonly T[] }): T;

const c = null as unknown as Carrier<Color>;

direct(c, { colors: ['red'] });           // ✅ narrows: T = Color
thunked(c, () => ({ colors: ['red'] }));  // ❌ string[] not assignable to readonly Color[]

🙁 Actual behavior

T is inferred as Color from Carrier<T> in both calls. In the thunked form the nested array literal ['red'] widens to string[] before contextual typing reaches it from readonly T[], producing Type 'string' is not assignable to type 'Color'.

🙂 Expected behavior

The thunked form narrows the same way as direct: ['red'] is contextually typed against readonly Color[] and the call type-checks. The two forms differ only in whether the options are bound directly or via an arrow function — the inferred T is the same.

Additional information about the issue

The pattern matters for reactive-resource wrapper libraries (e.g. glimmer-apollo) that require args to be wrapped in a function so they can re-evaluate when tracked state changes. The thunk shape is idiomatic; the inference asymmetry forces consumers to scatter as const casts across nested literals.

Possibly related, but verified not the same bug:

• Type inference regression on function calls not saved to variables, due to #61668 #62071 — same family (tuple inference inside arrow-returned composite), but a recent regression caused by Fix type variable leaks and cache inconsistencies #61668 (2025). This bug predates TS 4.9.
• Use tuple context based on apparent type of the contextual type uninstantiated by return mapper #62174 — closed PR proposing SkipReturnMapper fix for Type inference regression on function calls not saved to variables, due to #61668 #62071; doesn't apply to this code path.

[MartinJohns]

Duplicate of #241.

[johanrd]

Martin Johns (@MartinJohns) thanks