[libc++][NFC] Remove several redundant #if _LIBCPP_STD_VER > 17 in <span>

It turns out that the whole header is only enabled in C++20 and above,
so these checks were redundant (and always true).

Differential Revision: https://reviews.llvm.org/D121604

Author: ldionne

libcxx/include/span

@@ -158,7 +158,6 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 inline constexpr size_t dynamic_extent = numeric_limits<size_t>::max();
 template <typename _Tp, size_t _Extent = dynamic_extent> class span;
 
-
 template <class _Tp>
 struct __is_std_array : false_type {};
 
@@ -171,7 +170,7 @@ struct __is_std_span : false_type {};
 template <class _Tp, size_t _Sz>
 struct __is_std_span<span<_Tp, _Sz>> : true_type {};
 
-#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
+#if !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
 template <class _Range, class _ElementType>
 concept __span_compatible_range =
   ranges::contiguous_range<_Range> &&
@@ -181,7 +180,7 @@ concept __span_compatible_range =
   !__is_std_array<remove_cvref_t<_Range>>::value &&
   !is_array_v<remove_cvref_t<_Range>> &&
   is_convertible_v<remove_reference_t<ranges::range_reference_t<_Range>>(*)[], _ElementType(*)[]>;
-#endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
+#endif // !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
 
 template <typename _Tp, size_t _Extent>
 class _LIBCPP_TEMPLATE_VIS span {
@@ -211,7 +210,6 @@ public:
     constexpr span           (const span&) noexcept = default;
     constexpr span& operator=(const span&) noexcept = default;
 
-#if _LIBCPP_STD_VER > 17
     template <class _It,
               enable_if_t<contiguous_iterator<_It> &&
                               is_convertible_v<remove_reference_t<iter_reference_t<_It>>(*)[], element_type (*)[]>,
@@ -235,7 +233,6 @@ public:
       _LIBCPP_ASSERT(__last - __first == _Extent,
                      "invalid range in span's constructor (iterator, sentinel): last - first != extent");
     }
-#endif // _LIBCPP_STD_VER > 17
 
     _LIBCPP_INLINE_VISIBILITY constexpr span(type_identity_t<element_type> (&__arr)[_Extent]) noexcept : __data{__arr} {}
 
@@ -249,13 +246,13 @@ public:
     _LIBCPP_INLINE_VISIBILITY
     constexpr span(const array<_OtherElementType, _Extent>& __arr) noexcept : __data{__arr.data()} {}
 
-#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
+#if !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
     template <__span_compatible_range<element_type> _Range>
     _LIBCPP_INLINE_VISIBILITY
     constexpr explicit span(_Range&& __r) : __data{ranges::data(__r)} {
       _LIBCPP_ASSERT(ranges::size(__r) == _Extent, "size mismatch in span's constructor (range)");
     }
-#endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
+#endif
 
     template <class _OtherElementType>
     _LIBCPP_INLINE_VISIBILITY
@@ -402,7 +399,6 @@ public:
     constexpr span           (const span&) noexcept = default;
     constexpr span& operator=(const span&) noexcept = default;
 
-#if _LIBCPP_STD_VER > 17
     template <class _It,
               enable_if_t<contiguous_iterator<_It> &&
                               is_convertible_v<remove_reference_t<iter_reference_t<_It> > (*)[], element_type (*)[]>,
@@ -419,7 +415,6 @@ public:
     _LIBCPP_INLINE_VISIBILITY
     constexpr span(_It __first, _End __last)
         : __data(_VSTD::to_address(__first)), __size(__last - __first) {}
-#endif // _LIBCPP_STD_VER > 17
 
     template <size_t _Sz>
     _LIBCPP_INLINE_VISIBILITY
@@ -435,11 +430,11 @@ public:
     _LIBCPP_INLINE_VISIBILITY
     constexpr span(const array<_OtherElementType, _Sz>& __arr) noexcept : __data{__arr.data()}, __size{_Sz} {}
 
-#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
+#if !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
     template <__span_compatible_range<element_type> _Range>
     _LIBCPP_INLINE_VISIBILITY
     constexpr span(_Range&& __r) : __data(ranges::data(__r)), __size{ranges::size(__r)} {}
-#endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
+#endif
 
     template <class _OtherElementType, size_t _OtherExtent>
     _LIBCPP_INLINE_VISIBILITY
@@ -544,13 +539,11 @@ private:
     size_type __size;
 };
 
-#if _LIBCPP_STD_VER > 17
 template <class _Tp, size_t _Extent>
 inline constexpr bool ranges::enable_borrowed_range<span<_Tp, _Extent> > = true;
 
 template <class _ElementType, size_t _Extent>
 inline constexpr bool ranges::enable_view<span<_ElementType, _Extent>> = true;
-#endif // _LIBCPP_STD_VER > 17
 
 //  as_bytes & as_writable_bytes
 template <class _Tp, size_t _Extent>
@@ -579,7 +572,7 @@ template<class _Tp, size_t _Sz>
 template<class _Tp, size_t _Sz>
     span(const array<_Tp, _Sz>&) -> span<const _Tp, _Sz>;
 
-#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
+#if !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
 template<ranges::contiguous_range _Range>
     span(_Range&&) -> span<remove_reference_t<ranges::range_reference_t<_Range>>>;
 #endif