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Q&A TypeScript is unable to infer the correct type for a mapped tuple

I was playing around with mapped tuples and came across an interesting case where TypeScript cannot infer the types used: interface Foo<A, B> { (a: A, b: B): any } function test<...

0 answers · posted 2y ago by Moshi‭ · edited 2y ago by Moshi‭

Question typescript mapped-types type-inference

#2: Post edited by

Moshi‭ · 2022-04-22T20:16:39Z (almost 2 years ago)

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I was playing around with [mapped tuples](https://www.typescriptlang.org/docs/handbook/2/mapped-types.html) and came across an interesting case where TypeScript cannot infer the types used:
```typescript
interface Foo<A, B> {
(a: A, b: B): any
}
function test<A, Bs extends readonly any[]>(...args: { [K in keyof Bs]: Foo<A, Bs[K]> }) {}
let add: Foo<string, string> = (a, b) => a + b;
test(add);
```
```
error TS2345: Argument of type 'Foo<string, string>' is not assignable to parameter of type 'Foo<unknown, string>'.
Type 'unknown' is not assignable to type 'string'.
80 test(add);
~~~
```
Interestingly, this succeeds if `Foo` is an object interface; ex.
```typescript
interface Foo<A, B> {
a: A,
b: B,
}
~~function test<A, Bs extends readonly any[]>(...args: { [K in keyof Bs]: Foo<A, Bs[K]> }) {~~
~~// Do stuff~~
}
let foo: Foo<string, string> = {a: '', b: ''};
~~test(foo);~~
```
Thus, my question is two-fold:
1. Why does TypeScript fail to infer the correct type in the first case, where we pass in a function interface (Keep in mind that the concrete types for the passed argument should be known; they are even explicitly written.)
2. How can I fix this other than explicitly calling `test<string, string>(add)`?

I was playing around with [mapped tuples](https://www.typescriptlang.org/docs/handbook/2/mapped-types.html) and came across an interesting case where TypeScript cannot infer the types used:
```typescript
interface Foo<A, B> {
(a: A, b: B): any
}
function test<A, Bs extends readonly any[]>(...args: { [K in keyof Bs]: Foo<A, Bs[K]> }) {}
let add: Foo<string, string> = (a, b) => a + b;
test(add);
```
```
error TS2345: Argument of type 'Foo<string, string>' is not assignable to parameter of type 'Foo<unknown, string>'.
Type 'unknown' is not assignable to type 'string'.
80 test(add);
~~~
```
Interestingly, this succeeds if `Foo` is an object interface; ex.
```typescript
interface Foo<A, B> {
a: A,
b: B,
}
function test<A, Bs extends readonly any[]>(...args: { [K in keyof Bs]: Foo<A, Bs[K]> }): A {
throw "unimplemented";
}
let foo: Foo<string, string> = {a: '', b: ''};
let str = test(foo);
```
Though looking at my IDE, and adding a small [compile time assertion](https://abstract.properties/typescript-compile-assertions) shows that it actually can't correctly infer the type of this either; `A` (and thus `str`) has an inferred type of `unknown`, so the problem still shows up, it just doesn't immediately cause an error in this case.
```typescript
let string_assert: typeof str extends string ? true : never = true;
```
```
error TS2322: Type 'boolean' is not assignable to type 'never'.
13 let string_assert: typeof str extends string ? true : never = true;
```
Thus, my question is two-fold:
1. Why does TypeScript fail to infer the correct type in these cases? (Keep in mind that the concrete types for the passed argument should be known; they are even explicitly written.)
2. How can I fix this other than explicitly calling `test<string, string>(add)`?

#1: Initial revision by

Moshi‭ · 2022-04-22T04:19:59Z (almost 2 years ago)

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TypeScript is unable to infer the correct type for a mapped tuple

I was playing around with [mapped tuples](https://www.typescriptlang.org/docs/handbook/2/mapped-types.html) and came across an interesting case where TypeScript cannot infer the types used:

```typescript
interface Foo<A, B> {
    (a: A, b: B): any
}

function test<A, Bs extends readonly any[]>(...args: { [K in keyof Bs]: Foo<A, Bs[K]> }) {}

let add: Foo<string, string> = (a, b) => a + b;
test(add);
```

```
error TS2345: Argument of type 'Foo<string, string>' is not assignable to parameter of type 'Foo<unknown, string>'.
  Type 'unknown' is not assignable to type 'string'.

80 test(add);
        ~~~
```

Interestingly, this succeeds if `Foo` is an object interface; ex.

```typescript
interface Foo<A, B> {
	a: A,
	b: B,
}

function test<A, Bs extends readonly any[]>(...args: { [K in keyof Bs]: Foo<A, Bs[K]> }) {
	// Do stuff
}
let foo: Foo<string, string> = {a: '', b: ''};
test(foo);
```

Thus, my question is two-fold:

1. Why does TypeScript fail to infer the correct type in the first case, where we pass in a function interface (Keep in mind that the concrete types for the passed argument should be known; they are even explicitly written.)

2. How can I fix this other than explicitly calling `test<string, string>(add)`?

typescript mapped-types type-inference

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