Why this code is NOT causing redefinition error?-CodePudding

#include <initializer_list>

struct Foo
{
    template <typename T>
    Foo(std::initializer_list<T>) {}

    template <typename T>
    Foo(std::initializer_list<typename T::BarAlias>) {}
};

struct Bar
{
    using BarAlias = Bar; 
};

int main()
{
    Foo foo{ Bar{} };
}

I believe that a compiler should produce two exactly the same constructors inside of Foo. Why is it even working?

CodePudding user response：

You have two templates with unrelated template arguments Ts. For the second constructor to be a candidate, T should be deducible, at least. However, in

template <typename T>
Foo(std::initializer_list<typename T::BarAlias>) {}

T is in a non-deduced context. As a result, this constructor will always be rejected thanks to SFINAE.

Note how your code is different from

template<class T>
struct Foo {
    Foo(std::initializer_list<T>);
    Foo(std::initializer_list<typename T::BarAlias>);
};

which for Foo<Bar> foo{Bar{}}; will indeed produce an error that you expect.