Workaround MSVC 2019.8 bug with ranges::enable_view by including the definition

Summary: Owing to a set of bugs, it is not currently possible to actually declare ranges::enable_view without actually defining it, so we have to include the actual definition when we're in compilation units with it available. If the definition is not available, then the `const` form is sufficient to keep things compiling.

Reviewed By: yfeldblum

Differential Revision: D26337214

fbshipit-source-id: fb042b62967e3e6f536e8268e1b6dd6ccd20b810

folly/Range.h

@@ -1526,12 +1526,21 @@ FOLLY_POP_WARNING
 
 FOLLY_ASSUME_FBVECTOR_COMPATIBLE_1(folly::Range)
 
-// Tell the range-v3 library that this type should satisfy
-// the view concept (a lightweight, non-owning range).
+// Unfortunately it is not possible to forward declare enable_view under
+// MSVC 2019.8 due to compiler bugs, so we need to include the actual
+// definition if available.
+#if __has_include(<range/v3/range/concepts.hpp>) && defined(_MSC_VER) && _MSC_VER >= 1920
+#include <range/v3/range/concepts.hpp> // @manual
+#else
 namespace ranges {
 template <class T>
 extern const bool enable_view;
+} // namespace ranges
+#endif
 
+// Tell the range-v3 library that this type should satisfy
+// the view concept (a lightweight, non-owning range).
+namespace ranges {
 template <class Iter>
 FOLLY_INLINE_VARIABLE constexpr bool enable_view<::folly::Range<Iter>> = true;
 } // namespace ranges