Conditionally disabling a copy constructor

A noteworthy approach is partial specialization of the surrounding class template.

template <typename T,
          bool = std::is_copy_constructible<T>::value>
struct Foo
{
    T t;

    Foo() { /* ... */ }
    Foo(Foo const& other) : t(other.t) { /* ... */ }
};

template <typename T>
struct Foo<T, false> : Foo<T, true>
{
    using Foo<T, true>::Foo;

    // Now delete the copy constructor for this specialization:
    Foo(Foo const&) = delete;

    // These definitions adapt to what is provided in Foo<T, true>:
    Foo(Foo&&) = default;
    Foo& operator=(Foo&&) = default;
    Foo& operator=(Foo const&) = default;
};

This way the trait is_copy_constructible is satisfied exactly where T is_copy_constructible.