declare class Decoder<T> {
    decode(arrayBuffer: ArrayBuffer): T;
}

type ValueTypeOf<T extends Decoder<any>> = T extends Decoder<infer R> ? R : never;

type StructDescriptor = ReadonlyArray<readonly [key: string, type: Decoder<any>]>;

type StructTypeFor<Descriptor extends StructDescriptor> = {
    [K in (keyof Descriptor) & number as Descriptor[K][0]]: ValueTypeOf<Descriptor[K][1]>;
};

class StructDecoder<const Descriptor extends StructDescriptor> extends Decoder<StructTypeFor<Descriptor>> {
    constructor(descriptor: Descriptor) {
        super();
    }
}

declare const i32Decoder: Decoder<number>;
declare const i64Decoder: Decoder<bigint>;

const structDecoder = new StructDecoder([
    ["a", i32Decoder],
    ["b", i64Decoder],
]);

const struct = structDecoder.decode(new ArrayBuffer(100));

// I expected this would work, but it does not
const v: number = struct.a;