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Insufficient type narrowing of return type with Promise.all #35258
Description
Activity
another example:
declare class C<X> { } declare function fn<T1, T2>(values: readonly [C<T1>, C<T2>]): [T1, T2]; declare function x1(): C<number>; declare function x2(): C<number | null>; (async () => { const [a, b] = fn([x1(), x2()]); if (a < 1) { } })();
this code works if there is
- no readonly modifer
- or compiler option of strictNullChecks: false
- or no generic class
#33707 might fix this one
Reacted by Toni Villena, Christian Boehlke, Joseph Kohlmann and ExE BossThis issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.
- locked as resolved and limited conversation to collaborators
on Oct 21, 2025
TypeScript Version: 3.7.2
Search Terms:
generics readonly Promise.all
Code
{ "compilerOptions": { "target": "es2015", "module": "commonjs", "strictNullChecks": true, "esModuleInterop": true, } }Expected behavior:
type of
ashould benumberActual behavior:
type of
aisnumber | nullPlayground Link:
https://www.typescriptlang.org/play/index.html?noImplicitAny=false&strictFunctionTypes=false&strictPropertyInitialization=false&strictBindCallApply=false&noImplicitThis=false&noImplicitReturns=false&esModuleInterop=false&ssl=1&ssc=1&pln=8&pc=1#code/CYUwxgNghgTiAEAzArgOzAFwJYHtXwA8BGACgEoAueABRhwFssBnEAHlWXoCMQYA+ANwAoUJFgIU6bHkIAmclVoNmbDt17wAPvA4QIgoUJJQmAT3Txy8ALx94AbyEBIMHiYZ4AbSgAaeFwBdG3goAHcoLA8lRhYAOig9Ek9icj8CeTIAsmEnLERLKHhWeCIyB3gAXyEKsnJhIA
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