Proposed resolution:

This wording is relative to N5014.

1. In subclause [simd.loadstore] replace all occurrences of

   R(first, last)
   

   by

   R(first, static_cast<size_t>(last - first))
   

Currently, `simd::unchecked_load`/`partial_load`/`unchecked_store`/`partial_store` constructs a `span` via `span(first, last)` when taking an iterator-sentinel pair `first` and `last`.

However, the construction may not be well-behaved or well-formed when the sentinel type can be implicitly converted to an integer type. Consider:

struct I {
  using value_type = int;
  using difference_type = int;
  using iterator_category = std::contiguous_iterator_tag;
  // contiguous iterator operators
  // ...
 
  operator int() const;
};

int main() {
   std::simd::unchecked_load(I{}, I{});
}

Above, `unchecked_load` invokes `unchecked_load(I first, S last)` and we attempt to construct `span` through `span(first, last)`. However, this is invalid because the constructor requires that the sentinel type should not be convertible to `size_t`, so we fall back into `span(first, n)` via implicitly converting `I` to `size_t`. Such behavior is subtle and likely unintended.

Now consider:

struct I {
  using value_type = int;
  using difference_type = int;
  using iterator_category = std::contiguous_iterator_tag;
  // contiguous iterator operators
  // ...

  operator int() &&;
};

int main() {
   std::simd::unchecked_load(I{}, I{});
}

We still attempt to construct the `span` by calling `span(first, last)`, which is invalid, but because the lvalue sentinel cannot be converted to `size_t`, the call of `span(first, n)` is also invalid. This makes the construction of the `span` ill-formed and leads to a hard error in the function body.