Fix span iterator deduction guide (#61, thanks @Ryan-rsm-McKenzie)

README.md

@@ -430,6 +430,7 @@ span<>: Allows to construct from a non-null pointer to const and a size
 span<>: Allows to construct from a temporary pointer and a size
 span<>: Allows to construct from a temporary pointer to const and a size
 span<>: Allows to construct from any pointer and a zero size
+span<>: Allows to construct from an iterator and a size via an iterator deduction guide (C++17)
 span<>: Allows to construct from a C-array
 span<>: Allows to construct from a const C-array
 span<>: Allows to construct from a C-array with size via decay to pointer (potentially dangerous)

include/nonstd/span.hpp

@@ -661,8 +661,10 @@ using nonstd::byte;
 
 namespace std20 {
 
+#if span_HAVE( DEDUCTION_GUIDES )
 template< class T >
-struct iter_reference { typedef T type; };
+using iter_reference_t = decltype( *std::declval<T&>() );
+#endif
 
 } // namespace std20
 
@@ -1268,7 +1270,7 @@ span( Container const & ) -> span<const typename Container::value_type>;
 // iterator: constraints: It satisfies contiguous_­iterator.
 
 template< class It, class EndOrSize >
-span( It, EndOrSize ) -> span< typename std11::remove_reference< typename std20::iter_reference<It>::type >::type >;
+span( It, EndOrSize ) -> span< typename std11::remove_reference< typename std20::iter_reference_t<It> >::type >;
 
 #endif // span_HAVE( DEDUCTION_GUIDES )
 

test/span.t.cpp

@@ -248,6 +248,20 @@ CASE( "span<>: Allows to construct from any pointer and a zero size" )
     EXPECT_NO_THROW( F::nonnull() );
 }
 
+CASE( "span<>: Allows to construct from an iterator and a size via an iterator deduction guide (C++17)" )
+{
+#if span_HAVE( DEDUCTION_GUIDES )
+    char const * argv[] = { "prog", "arg1", "arg2" };
+    int  const   argc   = DIMENSION_OF( argv );
+
+    span args(argv, argc);
+
+    EXPECT( span.length() == DIMENSION_OF( argv ) );
+#else
+    EXPECT( !!"iterator deduction guide is not available (no C++17)" );
+#endif
+}
+
 CASE( "span<>: Allows to construct from a C-array" )
 {
     int arr[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, };