template <class T>
struct Foo
{
    T data;

    // ...

    template <class U>
    Foo(const Foo<U> &other) : data((T)doSomethingWith(other.data)) {}

    // ...
};

解决方法

A non-template constructor for class X is a copy constructor if its first parameter is of type X&,const X&,volatile X& or const volatile X&,and either there are no other parameters or else all other parameters have default arguments.

编译器仍将隐式生成默认值.

你可以明确(需要C 11)

Foo(const Foo<T>&) = default;