What causes this constructor to delegate to itself when it takes an initializer list and delegates a vector?

#include <initializer_list>
#include <vector>

struct test
{
    using t = std::vector<test>;
    test(t const &v)
    {
    }
    test(t &&v)
    {
    }
    test(std::initializer_list<test> v)
    : test{t{v}} //error
    {
    }
};

Both Clang and GCC complain that the third constructor, the one taking the initializer list, delegates to itself. I don't understand how this is possible though, because you can't construct an initializer list from a vector.

It is trivial to fix the error by replacing the outer curly braces with round parenthesis, but why would this be an issue in the first place? This almost identical program compiles just fine:

#include <initializer_list>

struct a {};
struct b {};

struct test
{
    test(a const &)
    {
    }
    test(a &&)
    {
    }
    test(std::initializer_list<b> v)
    : test{a{}}
    {
    }
};

Can someone explain what is going on here?