在 Typescript 中，强制泛型类型参数满足基于函数重载定义的类型约束

Question

下面给出了简化问题所需的代码框架。基本上，可以有不同类型的Source对象。为简单起见，下面的代码中显示了两种类型的源：DirectSource和IndirectSource。函数sourceAdapter()接受Source和其他帮助程序参数。

在示例中：如果缺少参数key，则Source必须是DirectSource;如果key是单个字符串值，则Source必须是IndirectSource，并且在这两种情况下，代码都会执行必要的调整并返回一个DirectSource对象。如果key是一个数组，则Source应该是IndirectSource这也是函数的返回值。

---

type KeyMap<T> = { [key in keyof T]: number };
type DirectSource = {
value: number;
otherFieldsAndMethods: any;
};
type IndirectSource<T extends object> = {
kvMap: KeyMap<T>;
otherFieldsAndMethods: any;
};
type Source<T extends number | object> = T extends object ? IndirectSource<T> : DirectSource;
// overloads
function sourceAdapter(src: Source<number>): DirectSource;
function sourceAdapter<T extends object>(src: Source<T>, key: keyof T): DirectSource;
function sourceAdapter<T extends object>(src: Source<T>, key: (keyof T)[]): IndirectSource<T>;
function sourceAdapter<T extends number | object>(
src: Source<T>,
key?: keyof T | (keyof T)[]
): T extends object ? IndirectSource<T> : DirectSource {
if (key) { // According to function overloads Source must be an IndirectSource
if (key instanceof Array) { // Config and return IndirectSource
const kvMap = key.reduce((ac, s) => {
ac[s] = (src as any).kvMap[s];
return ac;
}, {} as any);
// ******Error here:
// Type 'T' does not satisfy the constraint 'object'.
let ret: IndirectSource<T> = {
kvMap,
otherFieldsAndMethods: src.otherFieldsAndMethods
};
return ret;
} else { // Config and return DirectSource
let directSource = {
otherFieldsAndMethods: src.otherFieldsAndMethods,
value: (src as IndirectSource<any>).kvMap[key],
};
return directSource; // ******Error here: assignability
}
} else { // Source is a DirectSource, simply return the src.
return src;
}
}

---

导致错误的行用星号标记。这是通往操场的链接。

Answer 1

最简单最干净的方法是将 ReturnType 更改为联合。享受。 游乐场链接

type KeyMap<T> = { [key in keyof T]: number };
type DirectSource = {
value: number;
otherFieldsAndMethods: any;
};
type IndirectSource<T extends object> = {
kvMap: KeyMap<T>;
otherFieldsAndMethods: any;
};
type Source<T extends number | object> = T extends object ? IndirectSource<T> : DirectSource;
// overloads
function sourceAdapter(src: Source<number>): DirectSource;
function sourceAdapter<T extends object>(src: Source<T>, key: keyof T): DirectSource;
function sourceAdapter<T extends object>(src: Source<T>, key: (keyof T)[]): IndirectSource<T>;
function sourceAdapter<T extends object>(
src: Source<T>,
key?: keyof T | (keyof T)[]
): IndirectSource<T> | DirectSource {
if (key) { // According to function overloads Source must be an IndirectSource
if (key instanceof Array) { // Config and return IndirectSource
const kvMap = key.reduce((ac, s) => {
ac[s] = (src as any).kvMap[s];
return ac;
}, {} as any);
// Error here:
// Type 'T' does not satisfy the constraint 'object'.
let ret: IndirectSource<T> = {
kvMap,
otherFieldsAndMethods: src.otherFieldsAndMethods
};
return ret as IndirectSource<T>;
} else { // Config and return DirectSource
let directSource = {
otherFieldsAndMethods: src.otherFieldsAndMethods,
value: (src as IndirectSource<any>).kvMap[key],
};
return directSource; // Error here: assignability
}
} else { // Source is a DirectSource, simply return the src.
return src;
}
}