Prevent conversion constructions from absl::Span to itself

The conversions invoke the wrong constructor and merely happen to work correctly. However, they break trivial-copyability, and may also break if the constructor behaviors diverge (such as if a lifetime attribute is added).

PiperOrigin-RevId: 782932715
Change-Id: I280a03341887af241cf636c09f1c4318dc98fc16

Author: Abseil Team

absl/types/span.h

@@ -186,8 +186,9 @@ class ABSL_ATTRIBUTE_VIEW Span {
   // type C.
   template <typename C>
   using EnableIfConvertibleFrom =
-      typename std::enable_if<span_internal::HasData<T, C>::value &&
-                              span_internal::HasSize<C>::value>::type;
+      std::enable_if_t<!std::is_same_v<Span, std::remove_reference_t<C>> &&
+                       span_internal::HasData<T, C>::value &&
+                       span_internal::HasSize<C>::value>;
 
   // Used to SFINAE-enable a function when the slice elements are const.
   template <typename U>