TypeScript中为何string类型无法赋值给字符串联合类型？

定义的类型 #

我定义了以下字符串联合类型：

type FormFieldName =
    | "product details"
    | "objectives"
    | "stakeholders"
    | "functional requirements"
    | "non-functional requirements"
    | "assumptions and constraints"
    | "dependencies"
    | "acceptance criteria"
    | "risk analysis"
    | "priority effort"
    | "version history";

同时定义了PrdDataObject相关类型：

type PrdFormFieldsParamDefault = string | number | boolean | unknown[] | null;
type PrdFormFields<T> = {
    name: string;
    details: { [key: string]: T | PrdFormFields<T> | PrdFormFields<T>[] };
};

type PrdDataObject<
    T =
        | PrdFormFieldsParamDefault
        | Record<string, PrdFormFieldsParamDefault>
        | Record<string, PrdFormFieldsParamDefault>[],
> = {
    type: string | null;
    title: string | null;
    chosenLanguage: string | null;
    formFields: PrdFormFields<T>[] | null;
};

问题场景 #

我尝试为reduce方法中的累加器对象fieldsObject设置类型，代码如下：

const internalFormFields = prdDetails.formFields.reduce(
    (fieldsObject, field) => {
        const name: FormFieldName = field.name as FormFieldName;
        fieldsObject[name] = field.name;
        return fieldsObject;
    },
    {} as {
        [key in FormFieldName]: FormFieldName;
    }
);

执行到fieldsObject[name] = field.name;时，TypeScript报错：

Type 'string' is not assignable to type 'FormFieldName'

已尝试的方案 #

• 试过多种类型断言方式，均无效
• 认为类型守卫不是合适的解决方案
• 查阅过Stack Overflow相关问题，未找到适配的解决办法

内容的提问来源于stack exchange，提问作者Henry Ozoani