From 3b2419532e8cdf78a4ba585f4dd15b9e08a4a48f Mon Sep 17 00:00:00 2001
From: Neil MacIntosh <neilmac@microsoft.com>
Date: Mon, 12 Sep 2016 18:51:23 -0700
Subject: [PATCH] Corrected SFINAE for conversion constructors on span.

---
 gsl/span             | 31 +------------------------------
 tests/span_tests.cpp | 18 +++++++++---------
 2 files changed, 10 insertions(+), 39 deletions(-)

diff --git a/gsl/span b/gsl/span
index 8803ef1..cce8253 100644
--- a/gsl/span
+++ b/gsl/span
@@ -123,23 +123,6 @@ namespace details
     {
     };
 
-    template <class From, class To>
-    struct is_allowed_pointer_conversion
-        : public std::integral_constant<bool, std::is_pointer<From>::value &&
-                                                  std::is_pointer<To>::value &&
-                                                  std::is_convertible<From, To>::value>
-    {
-    };
-
-    template <class From, class To>
-    struct is_allowed_integral_conversion
-        : public std::integral_constant<
-              bool, std::is_integral<From>::value && std::is_integral<To>::value &&
-                        sizeof(From) == sizeof(To) && alignof(From) == alignof(To) &&
-                        std::is_convertible<From, To>::value>
-    {
-    };
-
     template <std::ptrdiff_t From, std::ptrdiff_t To>
     struct is_allowed_extent_conversion
         : public std::integral_constant<bool, From == To || From == gsl::dynamic_extent ||
@@ -150,19 +133,7 @@ namespace details
     template <class From, class To>
     struct is_allowed_element_type_conversion
         : public std::integral_constant<bool, std::is_same<From, std::remove_cv_t<To>>::value ||
-                                                  is_allowed_pointer_conversion<From, To>::value ||
-                                                  is_allowed_integral_conversion<From, To>::value>
-    {
-    };
-
-    template <class From>
-    struct is_allowed_element_type_conversion<From, byte>
-        : public std::integral_constant<bool, !std::is_const<From>::value>
-    {
-    };
-
-    template <class From>
-    struct is_allowed_element_type_conversion<From, const byte> : public std::true_type
+                                              std::is_convertible<From(*)[], To(*)[]>::value>
     {
     };
 
diff --git a/tests/span_tests.cpp b/tests/span_tests.cpp
index 571a821..a1dd64d 100644
--- a/tests/span_tests.cpp
+++ b/tests/span_tests.cpp
@@ -423,6 +423,7 @@ SUITE(span_tests)
             span<const int, 4> s{arr};
             CHECK(s.size() == narrow_cast<ptrdiff_t>(arr.size()) && s.data() == arr.data());
         }
+
 #ifdef CONFIRM_COMPILATION_ERRORS
         {
             span<const int, 2> s{arr};
@@ -437,6 +438,7 @@ SUITE(span_tests)
         {
             span<const int, 5> s{arr};
         }
+#endif
 
         {
             auto get_an_array = []() -> const std::array<int, 4> { return {1, 2, 3, 4}; };
@@ -444,7 +446,6 @@ SUITE(span_tests)
             // try to take a temporary std::array
             take_a_span(get_an_array());
         }
-#endif
     }
 
     TEST(from_std_array_const_constructor)
@@ -460,6 +461,7 @@ SUITE(span_tests)
             span<const int, 4> s{arr};
             CHECK(s.size() == narrow_cast<ptrdiff_t>(arr.size()) && s.data() == arr.data());
         }
+
 #ifdef CONFIRM_COMPILATION_ERRORS
         {
             span<const int, 2> s{arr};
@@ -509,7 +511,7 @@ SUITE(span_tests)
         {
 #ifdef CONFIRM_COMPILATION_ERRORS
             span<char> s{cstr};
-#endif
+#endif                                                          
             span<const char> cs{cstr};
             CHECK(cs.size() == narrow_cast<std::ptrdiff_t>(cstr.size()) &&
                   cs.data() == cstr.data());
@@ -520,7 +522,7 @@ SUITE(span_tests)
             auto get_temp_vector = []() -> std::vector<int> { return {}; };
             auto use_span = [](span<int> s) { static_cast<void>(s); };
             use_span(get_temp_vector());
-#endif
+#endif                      
         }
 
         {
@@ -534,7 +536,7 @@ SUITE(span_tests)
             auto get_temp_string = []() -> std::string { return{}; };
             auto use_span = [](span<char> s) { static_cast<void>(s); };
             use_span(get_temp_string());
-#endif
+#endif                         
         }
 
         {
@@ -552,11 +554,9 @@ SUITE(span_tests)
         }
 
         {
-#ifdef CONFIRM_COMPILATION_ERRORS
             auto get_temp_string = []() -> const std::string { return {}; };
             auto use_span = [](span<const char> s) { static_cast<void>(s); };
             use_span(get_temp_string());
-#endif
         }
 
         {
@@ -583,6 +583,7 @@ SUITE(span_tests)
 #endif
         }
 
+#ifdef CONFIRM_COMPILATION_ERRORS
         {
             span<int> s;
             span<unsigned int> s2 = s;
@@ -596,12 +597,11 @@ SUITE(span_tests)
         }
 
         {
-#ifdef CONFIRM_COMPILATION_ERRORS
             span<int> s;
             span<short> s2 = s;
             static_cast<void>(s2);
-#endif
         }
+#endif
     }
 
     TEST(copy_move_and_assignment)
@@ -689,7 +689,7 @@ SUITE(span_tests)
             span<int, 5> av = arr;
 #ifdef CONFIRM_COMPILATION_ERRORS
             CHECK(av.last<6>().length() == 6);
-#endif
+#endif    
             CHECK_THROW(av.last(6).length(), fail_fast);
         }