Let span_FEATURE_NON_MEMBER_FIRST_LAST_SUB make make_span() available (…

…#35, thanks to @Flamefire)

README.md

@@ -232,11 +232,11 @@ Define this to 1 to provide function `same()` to test if two spans refer as iden
 
 ### Provide `first()`, `last()` and `subspan()` functions
 -D<b>span_FEATURE_NON_MEMBER_FIRST_LAST_SUB</b>=1  
-Define this to 1 to provide functions `first()`, `last()` and `subspan()`. This requires `make_span()` functions to be available, see `span_FEATURE_MAKE_SPAN_TO_STD`. Default is undefined.
+Define this to 1 to provide functions `first()`, `last()` and `subspan()`. This implies `span_FEATURE_MAKE_SPAN` to provide functions `make_span()` that are required for this feature. Default is undefined.
 
 ### Provide `make_span()` functions
 -D<b>span_FEATURE_MAKE_SPAN</b>=1  
-Define this to 1 to provide creator functions `nonstd::make_span()`. Note that `span_FEATURE_MAKE_SPAN` takes precedence over `span_FEATURE_MAKE_SPAN_TO_STD`. Default is undefined.
+Define this to 1 to provide creator functions `nonstd::make_span()`. This feature is implied by using `span_FEATURE_NON_MEMBER_FIRST_LAST_SUB=1`. Note that `span_FEATURE_MAKE_SPAN` takes precedence over `span_FEATURE_MAKE_SPAN_TO_STD`. Default is undefined.
 
 -D<b>span_FEATURE_MAKE_SPAN_TO_STD</b>=14  
 Define this to the highest C++ language version for which to provide creator functions `nonstd::make_span()`, like 98, 03, 11, 14, 17, 20. Note that `span_FEATURE_MAKE_SPAN` takes precedence over `span_FEATURE_MAKE_SPAN_TO_STD`. Default is undefined.

include/nonstd/span.hpp

@@ -1162,13 +1162,10 @@ as_writeable_bytes( span<T,Extent> spn ) span_noexcept
 #endif // span_HAVE( BYTE ) || span_HAVE( NONSTD_BYTE )
 
 // extensions: non-member views:
+// this feature implies the presence of make_span()
 
 #if span_FEATURE( NON_MEMBER_FIRST_LAST_SUB ) && span_CPP11_120
 
-#if ! span_FEATURE( MAKE_SPAN )
-# error nonstd::span: non-member first(), last() and subspan() require make_span(), see span_FEATURE_MAKE_SPAN
-#endif
-
 template< extent_t Count, class T >
 span_constexpr auto
 first( T & t ) -> decltype( make_span(t).template first<Count>() )
@@ -1247,7 +1244,7 @@ using span_lite::same;
 
 // make_span() [span-lite extension]:
 
-#if span_FEATURE( MAKE_SPAN )
+#if span_FEATURE( MAKE_SPAN ) || span_FEATURE( NON_MEMBER_FIRST_LAST_SUB )
 
 namespace nonstd {
 namespace span_lite {