template operator visibility, gcc vs clang - c++

Question

I have this code, relying on a fold expression calling a very generic comparison operator:
#include <type_traits>
#include <vector>
#include <list>
template <typename Iter_a, typename Iter_b>
auto operator<(Iter_a, Iter_b) {
// original code makes more sense, I assure you
if constexpr ( std::is_same_v<Iter_a, Iter_b> ) {
return Iter_a();
}
else return Iter_b();
}
template <typename... Iterators>
using weakest_iterator = std::decay_t<decltype( (Iterators() < ...) )>;
int main() {
using lit = std::list<int>::iterator;
using vit = std::vector<int>::iterator;
using wi = weakest_iterator<lit, vit, float*>;
}
I have two problems with this code:
The first is that g++ compiles it and clang++ (-std=gnu++2a) refuses to do so:
prog.cc:14:64: error: call to function 'operator<' that is neither visible in the template definition nor found by argument-dependent lookup
using weakest_iterator = std::decay_t<decltype( (Iterators() < ...) )>;
^
prog.cc:19:16: note: in instantiation of template type alias 'weakest_iterator' requested here
using wi = weakest_iterator<lit, vit, float*>;
^
prog.cc:6:6: note: 'operator<' should be declared prior to the call site
auto operator<(Iter_a, Iter_b) {
Do you have any idea why? I find particularly troubling that clang requests a definition prior to the call site at line 19, that it finds by itself at line 6.
The second is that, if I modify it to invoke my meta-function with pointer template arguments (using wi = weakest_iterator<int*, float*, float*>;, I get another error message, this time only with g++ since clang refuses to compile, that I find difficult to really understand:
main.cpp: In substitution of 'template<class ... Iterators> using weakest_iterator = std::decay_t<decltype ((Iterators() < ...))> [with Iterators = {int*, float*, float*}]':
main.cpp:19:57: required from here
main.cpp:14:75: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
using weakest_iterator = std::decay_t<decltype( (Iterators() < ...) )>;
It seems that 1) the built-in overload of operator< is called (if there is anything such as this) over my ultra-generic one, and 2) that comparing an int* and a float* is considered the same as comparing a pointer and an integer.
Is it possible to make sure that my own implementation of operator< is chosen by the compiler?