Insertion to std::list<std::vector<int>> failed when using initializer list as the value?

Overload resolution for the

lv.insert(lv.end(), {});  // Oops!

call will resolve (formally, chosen as the best viable function as per [over.ics.rank]/3.1) to the following std::list<>::insert overload [extract from std::list<>::insert at cppreference, emphasis mine]:

iterator insert( const_iterator pos, std::initializer_list<T> ilist );

inserts elements from initializer list ilist before pos.

But as the initializer list is empty, there are no elements from it to be inserted.

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You could likewise invoke the same insert overload with a nested list initialization within the list initialization,

lv.insert(lv.end(), {{}});  // Size is now 2.

such that the innermost list initialization will resolve to (as per [over.match.list]/1) the std::initializer_list constructor of std::vector:

vector( std::initializer_list<T> init,
        const Allocator& alloc = Allocator() );

in so inserting a single element of type std::vector<int>, specifically an an empty such vector, into the std::list<std::vector<int>> object.