What is the purpose of std::type_identity in std::vector's constructors?

• other's type is actually const vector& which contains Allocator.
• alloc's type also contains Allocator.

Hence you have a situation similar to foo(1.0, 0). The designer of the ctor. decided, that Allocator should be deduced from other.

The following example will help understand the difference between using std::typeidentityt and not using it (online):

#include 
#include 
template 
struct MyVector {
    MyVector();
    MyVector(const MyVector& other, const Allocator& alloc);
    MyVector(MyVector&& other, const Allocator& alloc);
};
template  struct All1 : std::allocator {};
template  struct All2 : All1 {};
template  using PMA = std::pmr::polymorphicallocator;
template 
void Tryout(Vector v, A2 a)
{
    Vector v2(v, a);
}
int main()
{
    Tryout({}, {});
    Tryout({}, {});
    // error while constructing 'v2': template parameter 'Allocator' is ambiguous
    // Tryout({}, {});
    // Tryout({}, {});
}

The situation where std::typeidentityt makes a difference is when the ctor. call is CTAD and the 2^nd argument (the allocator) needs to be converted. A similar example is here: non-deduced contexts, although that is not CTAD.

std::typeidentity_t was added to the ctors. in P1518. Taken from n. m. could be an AI's comment.