Typescript 在映射类型中用作索引时，TypeScrip泛型类型参数无法正常工作

我无法理解使用泛型参数访问映射类型时的类型脚本行为，这里是触发错误的最简单代码:

export type mockInstances = {
  app: {
    id: number;
    code: string;
  };
  setup: {
    id: number;
    code: string;
  };
};

type MockEntityName = keyof mockInstances;

type MockInstance<EntityName extends MockEntityName> =
  mockInstances[EntityName];

const testWrapper = <EntityName extends MockEntityName>() => {
  //Errors
  const a: Partial<MockInstance<EntityName>> = {
    id: 1,
  };
  const b: Partial<MockInstances[EntityName]> = {
    id: 2,
  };
  console.log('ab', a, b);

  //Error: Type '{ id: 1; }' is not assignable to type 'Partial<MockInstance<EntityName>>'.ts(2322)

  //Works
  const c: Partial<MockInstance<'app'>> = {
    id: 1,
  };
  const d: Partial<MockInstances['app']> = {
    id: 2,
  };

  const e: Partial<MockInstances[mockEntityName]> = {
    id: 2,
  };

  console.log('cde', c, d, e);
};

我期望TypeScrip编译器能够使用EntityName参数(作为索引或类型参数)将a和b"赋值"给索引类型.

下面是这种奇怪的打字行为的简化版本:

export type mockInstances = {
  setup: {
    id: number;
    code: string;
    entityName: string;
    desc: string;
  };
};

type MockInstance<EntityName extends 'setup'> = mockInstances[EntityName];

const testWrapper = <EntityName extends 'setup'>(entityName: EntityName) => {
  //Error: Type '{ id: 1; code: "test"; entityName: EntityName; }'
  // is not assignable to type 'Partial<MockInstance<EntityName>>'.ts(2322)
  const a: Partial<MockInstance<EntityName>> = {
    id: 1,
    code: 'test',
    entityName,
  };

  const b: Partial<MockInstance<'setup'>> = {
    id: 1,
    code: 'test',
    entityName,
  };

  console.log(a, b);
};

所以我缩小了范围，在处理泛型时，Partial的行为并不像我期望的那样，我担心这可能会被认为是一个错误……

谢谢你对这个问题的任何启发.