Incorrect TS inference

wrong-ts-inference.ts

type RoleName = "admin" | "member";
type AdminPermission = "add-member" | "ban-member" | "delete-any-post";
type MemberPermission = "add-post" | "read-post" | "edit-post" | "delete-post";
type Permission = AdminPermission | MemberPermission;

type RoleToPermissions = {
  admin: readonly AdminPermission[];
  member: readonly MemberPermission[];
};

type Role<T extends RoleName> = Readonly<{
  role: T;
  permissions: RoleToPermissions[T];
}>;

type RolePermission = Role<"admin"> | Role<"member">;

// Internal RBAC library helper function
function getPermissionsForMember(): readonly MemberPermission[] {
  return ["add-post", "read-post", "edit-post", "delete-post"];
}

// Internal RBAC library helper function
function getPermissionsForAdmin(): readonly AdminPermission[] {
  return ["add-member", "ban-member", "delete-any-post"];
}

// Exposed RBAC library function
function getPermisisonsForRole<T extends RolePermission>(
  role: T["role"]
): T["permissions"] {
  return (
    {
      admin: getPermissionsForAdmin(),
      member: getPermissionsForAdmin(), // woops we accidentally used the wrong function, but get no error
    } as const
  )[role];
}

const memberPermissions = getPermisisonsForRole<Role<"member">>("member");
//    ^? const memberPermissions: readonly ["add-post", "read-post", "edit-post", "delete-post"]

// BOOM 💥 - our member now has admin permissions
console.log(memberPermissions); // ["add-member", "ban-member", "delete-any-post"]