Conditional type prevents assignability

Bug Report

🔎 Search Terms

Index signature assignability, generic index signatures, conditional types

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about conditional types

⏯ Playground Link

Playground link with relevant code

💻 Code

// Use of the conditional type `string extends T` prevents assignability.
// However, the conditional type `T extends string` is accepted.

declare class Foo<T>  {
    bar: string extends T ? number : boolean;
}

declare let indexable: Foo<{[key: string]: boolean}>;
declare let namedProp: Foo<{baz: boolean}>;

indexable = namedProp;

🙁 Actual behavior

When a conditional type is present in Foo, I sometimes cannot assign Foo<{a: any}> to Foo<[x: string]: any>, depending on the condition.

🙂 Expected behavior

Regardless of the conditional type, I should be able to assign Foo with named properties to Foo with an index signature.

[dylhunn]

Likely we’re considering a type extends T to mark Foo<T> as invariant in T

Is this on a roadmap or existing issue? I’d like to know whether to wait for this fix, or design around it. Thanks!

[RyanCavanaugh]

Types are not evaluated by iterating over each possible thing the type could be inhabited by, since doing things that way is combinatorially explosive. Things are generally either true for a category, or not, and an operation like this isn’t valid at a category level.