Fix return type of templated span.subspan() (microsoft#625)

* Added support for returning fixed-spize spans from subspan().

* Addressed issues from code review.

* Took simpler approach to static data member.

* Subtle fix to support MSVC 15.

* Helps to not introduce extraneous >

Author: neilmacintosh

include/gsl/span

@@ -45,8 +45,17 @@
 #if _MSC_VER < 1910
 #pragma push_macro("constexpr")
 #define constexpr /*constexpr*/
+#define GSL_USE_STATIC_CONSTEXPR_WORKAROUND
 
 #endif                          // _MSC_VER < 1910
+#else                           // _MSC_VER
+
+// See if we have enough C++17 power to use a static constexpr data member
+// without needing an out-of-line definition
+#if !(defined(__cplusplus) && (__cplusplus >= 201703L))
+#define GSL_USE_STATIC_CONSTEXPR_WORKAROUND
+#endif // !(defined(__cplusplus) && (__cplusplus >= 201703L))
+
 #endif                          // _MSC_VER
 
 #define GSL_NOEXCEPT noexcept
@@ -302,6 +311,12 @@ namespace details
     private:
         index_type size_;
     };
+
+    template <class ElementType, std::ptrdiff_t Extent, std::ptrdiff_t Offset, std::ptrdiff_t Count>
+    struct calculate_subspan_type
+    {
+      using type = span<ElementType, Count != dynamic_extent ? Count : (Extent != dynamic_extent ? Extent - Offset : Extent)>;
+    };
 } // namespace details
 
 // [span], class template span
@@ -323,7 +338,11 @@ public:
 
     using size_type = index_type;
 
-    constexpr static const index_type extent = Extent;
+#if defined(GSL_USE_STATIC_CONSTEXPR_WORKAROUND)
+    static constexpr const index_type extent { Extent };
+#else
+    static constexpr index_type extent { Extent };
+#endif
 
     // [span.cons], span constructors, copy, assignment, and destructor
     template <bool Dependent = false,
@@ -412,7 +431,7 @@ public:
     }
 
     template <std::ptrdiff_t Offset, std::ptrdiff_t Count = dynamic_extent>
-    constexpr span<element_type, Count> subspan() const
+    constexpr auto subspan() const -> typename details::calculate_subspan_type<ElementType, Extent, Offset, Count>::type
     {
         Expects((Offset >= 0 && size() - Offset >= 0) &&
                 (Count == dynamic_extent || (Count >= 0 && Offset + Count <= size())));
@@ -525,6 +544,11 @@ private:
     }
 };
 
+#if defined(GSL_USE_STATIC_CONSTEXPR_WORKAROUND)
+template <class ElementType, std::ptrdiff_t Extent>
+constexpr const typename span<ElementType, Extent>::index_type span<ElementType, Extent>::extent;
+#endif
+
 
 // [span.comparison], span comparison operators
 template <class ElementType, std::ptrdiff_t FirstExtent, std::ptrdiff_t SecondExtent>

tests/span_tests.cpp

@@ -776,35 +776,47 @@ TEST_CASE("subspan")
     {
         span<int, 5> av = arr;
         CHECK((av.subspan<2, 2>().size() == 2));
+        CHECK(decltype(av.subspan<2, 2>())::extent == 2);
         CHECK(av.subspan(2, 2).size() == 2);
         CHECK(av.subspan(2, 3).size() == 3);
     }
 
     {
         span<int, 5> av = arr;
         CHECK((av.subspan<0, 0>().size() == 0));
+        CHECK(decltype(av.subspan<0,0>())::extent == 0);
         CHECK(av.subspan(0, 0).size() == 0);
     }
 
     {
         span<int, 5> av = arr;
         CHECK((av.subspan<0, 5>().size() == 5));
+        CHECK(decltype(av.subspan<0, 5>())::extent == 5);
         CHECK(av.subspan(0, 5).size() == 5);
+
         CHECK_THROWS_AS(av.subspan(0, 6).size(), fail_fast);
         CHECK_THROWS_AS(av.subspan(1, 5).size(), fail_fast);
     }
 
     {
         span<int, 5> av = arr;
         CHECK((av.subspan<4, 0>().size() == 0));
+        CHECK(decltype(av.subspan<4, 0>())::extent == 0);
         CHECK(av.subspan(4, 0).size() == 0);
         CHECK(av.subspan(5, 0).size() == 0);
         CHECK_THROWS_AS(av.subspan(6, 0).size(), fail_fast);
     }
 
+    {
+        span<int, 5> av = arr;
+        CHECK((av.subspan<1>().size() == 4));
+        CHECK(decltype(av.subspan<1>())::extent == 4);    
+    }
+
     {
         span<int> av;
         CHECK((av.subspan<0, 0>().size() == 0));
+        CHECK((decltype(av.subspan<0, 0>())::extent == 0));
         CHECK(av.subspan(0, 0).size() == 0);
         CHECK_THROWS_AS((av.subspan<1, 0>().size()), fail_fast);
     }