[FIX] simplify std::ranges::enable_view overloads

I know that this introduces a macro (SEQAN3_STD_RANGES_NAMESPACE), but
this reduces code bloat for a feature that is only used for a single
data structure right now.

This PR is also in line with #1678 to remove dependencies of our library
in the std headers.

include/seqan3/range/decorator/gap_decorator.hpp

@@ -889,14 +889,14 @@ class gap_decorator<inner_type>::gap_decorator_iterator
 
 } // namespace seqan
 
-namespace seqan3::detail
+namespace SEQAN3_STD_RANGES_NAMESPACE
 {
 
 //!\brief Type trait that declares any seqan3::gap_decorator to be **NOT a view**.
 template <typename type>
-constexpr int enable_view<seqan3::gap_decorator<type>> = 0;
+constexpr bool enable_view<seqan3::gap_decorator<type>> = false;
 
 template <typename type>
-constexpr int enable_view<seqan3::gap_decorator<type> const> = 0;
+constexpr bool enable_view<seqan3::gap_decorator<type> const> = false;
 
-} // namespace seqan3::detail
+} // namespace std::ranges

include/seqan3/std/ranges

@@ -13,7 +13,20 @@
 
 #pragma once
 
-#include <seqan3/range/detail/enable_view.hpp>
+/*!\brief The namespace where all ranges are defined (i.e., std::ranges), this is needed for partial specialisation's
+ *        like std::ranges::enable_view
+ *
+ * \details In this shim header we import ranges::cpp20 if the compiler does not support std::ranges natively. This
+ *          leads to the problem that customisation points like std::ranges::enable_view are declared in different
+ *          namespace (e.g. ranges::enable_view when shimmed), but the compiler disallows to partial specialise
+ *          std::ranges::enable_view if it was declared in a different namespace. Thus, this macro stores in which
+ *          namespace customisation points like enable_view were defined.
+ */
+#if __cpp_lib_ranges
+#define SEQAN3_STD_RANGES_NAMESPACE std::ranges
+#else
+#define SEQAN3_STD_RANGES_NAMESPACE ranges
+#endif
 
 #if __cpp_lib_ranges // C++20 ranges available
 #include <ranges>
@@ -76,36 +89,4 @@ namespace views = ranges::views;
 
 } // namespace std::
 
-// ============================================================================
-//  traits voodoo
-// ============================================================================
-
-namespace ranges
-{
-
-//!\brief Customises ranges type trait that indicates whether `type` is a view.
-template <typename type>
-//!\cond
-    requires (seqan3::detail::enable_view<type> == 0) || (seqan3::detail::enable_view<type> == 1)
-//!\endcond
-constexpr bool enable_view<type> = static_cast<bool>(seqan3::detail::enable_view<type>);
-
-} // namespace ranges
-
-#endif // no standard header
-
-#if __cpp_lib_ranges
-
-namespace std::ranges
-{
-
-//!\brief Customises std::ranges type trait that indicates whether `type` is a view.
-template <typename type>
-//!\cond
-    requires (seqan3::detail::enable_view<type> == 0) || (seqan3::detail::enable_view<type> == 1)
-//!\endcond
-constexpr bool enable_view<type> = static_cast<bool>(seqan3::detail::enable_view<type>);
-
-} // namespace std::ranges
-
 #endif // standard header