Why is implicit conversion from pair<int64_t,int64_t> to pair<int,int> allowed?

Question

I've noticed some cases where std::pair will be implicitly converted to a std::pair of a different type without warning, but I can't figure out why. I've had a look through the constructors for std::pair, and can't figure it out. Can somebody explain this to me?

Example:

 std::pair<int,int> a = std::pair<int64_t, int64_t>(1,2);

Gives no warnings with clang -Wall -Weverything.

Answer 1

The constructor

template<class U, class V> pair(const pair<U, V>& p);

allows copying from any pairs provided the first element of p can be converted to first element of pair being constructed, and the same for second elements.