The CTAD of the standard container is still instantiated when Alloc does not satisfy Allocator.

Not sure if this is a language bug or a library bug:

https://godbolt.org/z/EY6K9eKP9

#include <vector>
#include <list>

template<template <class...> class Cnt, class Alloc>
concept can_ctad = requires (int* it) {
  Cnt(it, it, Alloc{});
};

struct NotAlloc { };

static_assert(!can_ctad<std::vector, NotAlloc>); // hard error in MSVC-STL
static_assert(!can_ctad<std::list, NotAlloc>); // hard error in MSVC-STL

Start with the Godbolt reproducer and the static_assert cases for std::vector and std::list, then inspect the corresponding standard-container deduction guides and allocator constraints. Done means determining whether the hard error is conforming and, if not, adding a regression test that makes both can_ctad assertions compile successfully.