From c366f4415d581b03e2a900cf36c40e830cd05bf9 Mon Sep 17 00:00:00 2001
From: Neil MacIntosh <neilmac@microsoft.com>
Date: Tue, 26 Jul 2016 18:34:27 -0700
Subject: [PATCH] Fixed compilation issues for GCC on Linux.

---
 include/gsl_util.h          |   1 -
 include/span.h              | 207 +++++++++++++++--------------
 include/string_span.h       | 258 +++++++++++++-----------------------
 tests/string_span_tests.cpp |   6 +-
 4 files changed, 207 insertions(+), 265 deletions(-)

diff --git a/include/gsl_util.h b/include/gsl_util.h
index a3d5fbe..92a795b 100644
--- a/include/gsl_util.h
+++ b/include/gsl_util.h
@@ -118,7 +118,6 @@ inline T narrow(U u)
 {
     T t = narrow_cast<T>(u);
     if (static_cast<U>(t) != u) throw narrowing_error();
-#pragma warning(suppress : 4127) // suppress warning from MSVC compiler about constant in if-test
     if (!details::is_same_signedness<T, U>::value && ((t < T{}) != (u < U{})))
         throw narrowing_error();
     return t;
diff --git a/include/span.h b/include/span.h
index f40a7ab..27c479d 100644
--- a/include/span.h
+++ b/include/span.h
@@ -79,12 +79,12 @@
 namespace gsl
 {
 
-template <class ElementType, std::ptrdiff_t Extent = dynamic_extent>
-class span;
-
 // [views.constants], constants
 constexpr const std::ptrdiff_t dynamic_extent = -1;
 
+template <class ElementType, std::ptrdiff_t Extent = dynamic_extent>
+class span;
+
 // implementation details
 namespace details
 {
@@ -99,20 +99,35 @@ namespace details
     };
 
     template <class T>
-    struct is_span : is_span_oracle<std::remove_cv_t<T>>
+    struct is_span : public is_span_oracle<std::remove_cv_t<T>>
+    {
+    };
+
+    template <class T>
+    struct is_std_array_oracle : std::false_type
+    {
+    };
+
+    template <class ElementType, size_t Extent>
+    struct is_std_array_oracle<std::array<ElementType, Extent>> : std::true_type
+    {
+    };
+
+    template <class T>
+    struct is_std_array : public is_std_array_oracle<std::remove_cv_t<T>>
     {
     };
 
     template <class From, class To>
     struct is_allowed_pointer_conversion
-        : std::bool_constant<std::is_pointer<From>::value && std::is_pointer<To>::value &&
+        : 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
-        : std::bool_constant<std::is_integral<From>::value && std::is_integral<To>::value &&
+        : 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>
     {
@@ -120,13 +135,13 @@ namespace details
 
     template <std::ptrdiff_t From, std::ptrdiff_t To>
     struct is_allowed_extent_conversion
-        : std::bool_constant<From == To || From == gsl::dynamic_extent || To == gsl::dynamic_extent>
+        : public std::integral_constant<bool, From == To || From == gsl::dynamic_extent || To == gsl::dynamic_extent>
     {
     };
 
     template <class From, class To>
     struct is_allowed_element_type_conversion
-        : std::bool_constant<std::is_same<From, std::remove_cv_t<To>>::value ||
+        : 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>
     {
@@ -134,12 +149,12 @@ namespace details
 
     template <class From>
     struct is_allowed_element_type_conversion<From, byte>
-        : std::bool_constant<!std::is_const<From>::value>
+        : public std::integral_constant<bool, !std::is_const<From>::value>
     {
     };
 
     template <class From>
-    struct is_allowed_element_type_conversion<From, const byte> : std::true_type
+    struct is_allowed_element_type_conversion<From, const byte> : public std::true_type
     {
     };
 
@@ -269,12 +284,12 @@ namespace details
         void swap(const_span_iterator& rhs) noexcept
         {
             std::swap(index_, rhs.index_);
-            std::swap(m_span, rhs.m_span);
+            std::swap(span_, rhs.span_);
         }
 
     private:
         const Span* span_;
-        ptrdiff_t index_;
+        std::ptrdiff_t index_;
     };
 
     template <class Span>
@@ -323,12 +338,12 @@ namespace details
 
         constexpr span_iterator operator+(difference_type n) const noexcept
         {
-            return base_type::operator+(n);
+            return {base_type::operator+(n)};
         }
 
         constexpr span_iterator& operator+=(difference_type n) noexcept
         {
-            return base_type::operator+=(n);
+            return {base_type::operator+=(n)};
         }
 
         constexpr span_iterator operator-(difference_type n) const noexcept
@@ -376,6 +391,10 @@ namespace details
         }
 
         void swap(span_iterator& rhs) noexcept { base_type::swap(rhs); }
+    private:
+        constexpr span_iterator(const base_type& base) : base_type(base)
+        {
+        }
     };
 
     template <typename Span>
@@ -408,6 +427,47 @@ namespace details
         return rhs - n;
     }
 
+    template <std::ptrdiff_t Ext>
+    class extent_type
+    {
+    public:
+	using index_type = std::ptrdiff_t;
+
+        static_assert(Ext >= 0, "A fixed-size span must be >= 0 in size.");
+
+        constexpr extent_type() noexcept {}
+
+        template <index_type Other>
+        constexpr extent_type(extent_type<Other> ext) noexcept
+        {
+            static_assert(Other == Ext || Other == dynamic_extent,
+                          "Mismatch between fixed-size extent and size of initializing data.");
+            Expects(ext.size() == Ext);
+        }
+
+        constexpr extent_type(index_type size) { Expects(size == Ext); }
+
+        constexpr inline index_type size() const noexcept { return Ext; }
+    };
+
+    template <>
+    class extent_type<dynamic_extent>
+    {
+    public:
+	using index_type = std::ptrdiff_t;
+
+        template <index_type Other>
+        explicit constexpr extent_type(extent_type<Other> ext) : size_(ext.size())
+        {
+        }
+
+        explicit constexpr extent_type(index_type size) : size_(size) { Expects(size >= 0); }
+
+        constexpr inline index_type size() const noexcept { return size_; }
+
+    private:
+        index_type size_;
+    };
 } // namespace details
 
 // [span], class template span
@@ -429,9 +489,9 @@ public:
     constexpr static const index_type extent = Extent;
 
     // [span.cons], span constructors, copy, assignment, and destructor
-    constexpr span() noexcept : storage_(nullptr, extent_type<0>()) {}
+    constexpr span() noexcept : storage_(nullptr, details::extent_type<0>()) {}
 
-    constexpr span(nullptr_t) noexcept : span() {}
+    constexpr span(std::nullptr_t) noexcept : span() {}
 
     constexpr span(pointer ptr, index_type count) : storage_(ptr, count) {}
 
@@ -441,24 +501,19 @@ public:
     }
 
     template <size_t N>
-    constexpr span(element_type (&arr)[N]) noexcept : storage_(&arr[0], extent_type<N>())
+    constexpr span(element_type (&arr)[N]) noexcept : storage_(&arr[0], details::extent_type<N>())
+    {
+    }
+    
+    template <size_t N, class ArrayElementType = std::remove_const_t<element_type>>
+    constexpr span(std::array<ArrayElementType, N>& arr) noexcept
+        : storage_(&arr[0], details::extent_type<N>())
     {
     }
 
     template <size_t N>
-    constexpr span(std::array<element_type, N>& arr) noexcept : storage_(&arr[0], extent_type<N>())
-    {
-    }
-
-    template <size_t N, class = std::enable_if_t<is_const<element_type>::value>>
-    constexpr span(std::array<std::remove_const_t<element_type>, N>& arr) noexcept
-        : storage_(&arr[0], extent_type<N>())
-    {
-    }
-
-    template <size_t N, class = std::enable_if_t<is_const<element_type>::value>>
     constexpr span(const std::array<std::remove_const_t<element_type>, N>& arr) noexcept
-        : storage_(&arr[0], extent_type<N>())
+        : storage_(&arr[0], details::extent_type<N>())
     {
     }
 
@@ -467,8 +522,9 @@ public:
     template <class Container,
               class = std::enable_if_t<
                   !details::is_span<Container>::value &&
-                  std::is_convertible<Container::pointer, pointer>::value &&
-                  std::is_convertible<Container::pointer,
+                  !details::is_std_array<Container>::value && 
+                  std::is_convertible<typename Container::pointer, pointer>::value &&
+                  std::is_convertible<typename Container::pointer,
                                       decltype(std::declval<Container>().data())>::value>>
     constexpr span(Container& cont) : span(cont.data(), cont.size())
     {
@@ -477,8 +533,8 @@ public:
     template <class Container,
               class = std::enable_if_t<
                   std::is_const<element_type>::value && !details::is_span<Container>::value &&
-                  std::is_convertible<Container::pointer, pointer>::value &&
-                  std::is_convertible<Container::pointer,
+                  std::is_convertible<typename Container::pointer, pointer>::value &&
+                  std::is_convertible<typename Container::pointer,
                                       decltype(std::declval<Container>().data())>::value>>
     constexpr span(const Container& cont) : span(cont.data(), cont.size())
     {
@@ -493,7 +549,7 @@ public:
             details::is_allowed_extent_conversion<OtherExtent, Extent>::value &&
             details::is_allowed_element_type_conversion<OtherElementType, element_type>::value>>
     constexpr span(const span<OtherElementType, OtherExtent>& other)
-        : storage_(reinterpret_cast<pointer>(other.data()), extent_type<OtherExtent>(other.size()))
+        : storage_(reinterpret_cast<pointer>(other.data()), details::extent_type<OtherExtent>(other.size()))
     {
     }
 
@@ -503,7 +559,7 @@ public:
             details::is_allowed_extent_conversion<OtherExtent, Extent>::value &&
             details::is_allowed_element_type_conversion<OtherElementType, element_type>::value>>
     constexpr span(span<OtherElementType, OtherExtent>&& other)
-        : storage_(reinterpret_cast<pointer>(other.data()), extent_type<OtherExtent>(other.size()))
+        : storage_(reinterpret_cast<pointer>(other.data()), details::extent_type<OtherExtent>(other.size()))
     {
     }
 
@@ -512,25 +568,25 @@ public:
     constexpr span& operator=(span&& other) noexcept = default;
 
     // [span.sub], span subviews
-    template <ptrdiff_t Count>
+    template <std::ptrdiff_t Count>
     constexpr span<element_type, Count> first() const
     {
         Expects(Count >= 0 && Count <= size());
         return {data(), Count};
     }
 
-    template <ptrdiff_t Count>
+    template <std::ptrdiff_t Count>
     constexpr span<element_type, Count> last() const
     {
         Expects(Count >= 0 && Count <= size());
         return {data() + (size() - Count), Count};
     }
 
-    template <ptrdiff_t Offset, ptrdiff_t Count = dynamic_extent>
+    template <std::ptrdiff_t Offset, std::ptrdiff_t Count = dynamic_extent>
     constexpr span<element_type, Count> subspan() const
     {
-        Expects((Offset == 0 || Offset > 0 && Offset <= size()) &&
-                (Count == dynamic_extent || Count >= 0 && Offset + Count <= size()));
+        Expects((Offset == 0 || (Offset > 0 && Offset <= size())) &&
+                (Count == dynamic_extent || (Count >= 0 && Offset + Count <= size())));
         return {data() + Offset, Count == dynamic_extent ? size() - Offset : Count};
     }
 
@@ -549,8 +605,8 @@ public:
     constexpr span<element_type, dynamic_extent> subspan(index_type offset,
                                                          index_type count = dynamic_extent) const
     {
-        Expects((offset == 0 || offset > 0 && offset <= size()) &&
-                (count == dynamic_extent || count >= 0 && offset + count <= size()));
+        Expects((offset == 0 || (offset > 0 && offset <= size())) &&
+                (count == dynamic_extent || (count >= 0 && offset + count <= size())));
         return {data() + offset, count == dynamic_extent ? size() - offset : count};
     }
 
@@ -584,47 +640,6 @@ public:
     const_reverse_iterator crend() const noexcept { return reverse_iterator{{this, 0}}; }
 
 private:
-    template <index_type Extent>
-    class extent_type;
-
-    template <index_type Extent>
-    class extent_type
-    {
-    public:
-        static_assert(Extent >= 0, "A fixed-size span must be >= 0 in size.");
-
-        constexpr extent_type() noexcept {}
-
-        template <index_type Other>
-        constexpr extent_type(extent_type<Other> ext) noexcept
-        {
-            static_assert(Other == Extent || Other == dynamic_extent,
-                          "Mismatch between fixed-size extent and size of initializing data.");
-            Expects(ext.size() == Extent);
-        }
-
-        constexpr extent_type(index_type size) { Expects(size == Extent); }
-
-        constexpr inline index_type size() const noexcept { return Extent; }
-    };
-
-    template <>
-    class extent_type<dynamic_extent>
-    {
-    public:
-        template <index_type Other>
-        explicit constexpr extent_type(extent_type<Other> ext) : size_(ext.size())
-        {
-        }
-
-        explicit constexpr extent_type(index_type size) : size_(size) { Expects(size >= 0); }
-
-        constexpr inline index_type size() const noexcept { return size_; }
-
-    private:
-        index_type size_;
-    };
-
     // this implementation detail class lets us take advantage of the
     // empty base class optimization to pay for only storage of a single
     // pointer in the case of fixed-size spans
@@ -644,41 +659,41 @@ private:
         pointer data_;
     };
 
-    storage_type<extent_type<Extent>> storage_;
+    storage_type<details::extent_type<Extent>> storage_;
 };
 
 // [span.comparison], span comparison operators
-template <class ElementType, ptrdiff_t Extent>
-constexpr bool operator==(const span<ElementType, Extent>& l, const span<ElementType, Extent>& r)
+template <class ElementType, std::ptrdiff_t FirstExtent, std::ptrdiff_t SecondExtent>
+constexpr bool operator==(const span<ElementType, FirstExtent>& l, const span<ElementType, SecondExtent>& r)
 {
     return std::equal(l.begin(), l.end(), r.begin(), r.end());
 }
 
-template <class ElementType, ptrdiff_t Extent>
+template <class ElementType, std::ptrdiff_t Extent>
 constexpr bool operator!=(const span<ElementType, Extent>& l, const span<ElementType, Extent>& r)
 {
     return !(l == r);
 }
 
-template <class ElementType, ptrdiff_t Extent>
+template <class ElementType, std::ptrdiff_t Extent>
 constexpr bool operator<(const span<ElementType, Extent>& l, const span<ElementType, Extent>& r)
 {
     return std::lexicographical_compare(l.begin(), l.end(), r.begin(), r.end());
 }
 
-template <class ElementType, ptrdiff_t Extent>
+template <class ElementType, std::ptrdiff_t Extent>
 constexpr bool operator<=(const span<ElementType, Extent>& l, const span<ElementType, Extent>& r)
 {
     return !(l > r);
 }
 
-template <class ElementType, ptrdiff_t Extent>
+template <class ElementType, std::ptrdiff_t Extent>
 constexpr bool operator>(const span<ElementType, Extent>& l, const span<ElementType, Extent>& r)
 {
     return r < l;
 }
 
-template <class ElementType, ptrdiff_t Extent>
+template <class ElementType, std::ptrdiff_t Extent>
 constexpr bool operator>=(const span<ElementType, Extent>& l, const span<ElementType, Extent>& r)
 {
     return !(l < r);
@@ -692,11 +707,11 @@ namespace details
     // we should use a narrow_cast<> to go to size_t, but older compilers may not see it as
     // constexpr
     // and so will fail compilation of the template
-    template <class ElementType, ptrdiff_t Extent>
+    template <class ElementType, std::ptrdiff_t Extent>
     struct calculate_byte_size
         : std::integral_constant<std::ptrdiff_t,
-                                 static_cast<ptrdiff_t>(sizeof(ElementType) *
-                                                        static_cast<size_t>(Extent))>
+                                 static_cast<std::ptrdiff_t>(sizeof(ElementType) *
+                                                        static_cast<std::size_t>(Extent))>
     {
     };
 
@@ -708,14 +723,14 @@ namespace details
 }
 
 // [span.objectrep], views of object representation
-template <class ElementType, ptrdiff_t Extent>
+template <class ElementType, std::ptrdiff_t Extent>
 span<const byte, details::calculate_byte_size<ElementType, Extent>::value>
 as_bytes(span<ElementType, Extent> s) noexcept
 {
     return {reinterpret_cast<const byte*>(s.data()), s.size_bytes()};
 }
 
-template <class ElementType, ptrdiff_t Extent,
+template <class ElementType, std::ptrdiff_t Extent,
           class = std::enable_if_t<!std::is_const<ElementType>::value>>
 span<byte, details::calculate_byte_size<ElementType, Extent>::value>
 as_writeable_bytes(span<ElementType, Extent> s) noexcept
diff --git a/include/string_span.h b/include/string_span.h
index bef8288..723f650 100644
--- a/include/string_span.h
+++ b/include/string_span.h
@@ -234,40 +234,39 @@ template <typename CharT, std::ptrdiff_t Extent = dynamic_extent>
 class basic_string_span
 {
 public:
-    using value_type = CharT;
-    using const_value_type = std::add_const_t<value_type>;
-    using pointer = std::add_pointer_t<value_type>;
-    using reference = std::add_lvalue_reference_t<value_type>;
-    using const_reference = std::add_lvalue_reference_t<const_value_type>;
-    using impl_type = span<value_type, Extent>;
+    using element_type = CharT;
+    using pointer = std::add_pointer_t<element_type>;
+    using reference = std::add_lvalue_reference_t<element_type>;
+    using const_reference = std::add_lvalue_reference_t<std::add_const_t<element_type>>;
+    using impl_type = span<element_type, Extent>;
 
-    using size_type = ptrdiff_t;
+    using index_type = typename impl_type::index_type;
     using iterator = typename impl_type::iterator;
     using const_iterator = typename impl_type::const_iterator;
     using reverse_iterator = typename impl_type::reverse_iterator;
     using const_reverse_iterator = typename impl_type::const_reverse_iterator;
 
     // default (empty)
-    constexpr basic_string_span() = default;
+    constexpr basic_string_span() noexcept = default;
 
     // copy
-    constexpr basic_string_span(const basic_string_span& other) = default;
+    constexpr basic_string_span(const basic_string_span& other) noexcept = default;
 
 // move
 #ifndef GSL_MSVC_NO_DEFAULT_MOVE_CTOR
-    constexpr basic_string_span(basic_string_span&& other) = default;
+    constexpr basic_string_span(basic_string_span&& other) noexcept = default;
 #else
     constexpr basic_string_span(basic_string_span&& other) : span_(std::move(other.span_)) {}
 #endif
 
     // assign
-    constexpr basic_string_span& operator=(const basic_string_span& other) = default;
+    constexpr basic_string_span& operator=(const basic_string_span& other) noexcept = default;
 
 // move assign
 #ifndef GSL_MSVC_NO_DEFAULT_MOVE_CTOR
-    constexpr basic_string_span& operator=(basic_string_span&& other) = default;
+    constexpr basic_string_span& operator=(basic_string_span&& other) noexcept = default;
 #else
-    constexpr basic_string_span& operator=(basic_string_span&& other)
+    constexpr basic_string_span& operator=(basic_string_span&& other) noexcept
     {
         span_ = std::move(other.span_);
         return *this;
@@ -277,47 +276,55 @@ public:
     // from nullptr
     constexpr basic_string_span(std::nullptr_t ptr) noexcept : span_(ptr) {}
 
-    // from nullptr and length
-    constexpr basic_string_span(std::nullptr_t ptr, size_type length) noexcept : span_(ptr, length)
-    {
-    }
+    constexpr basic_string_span(pointer ptr, index_type length) : span_(ptr, length) {}
+    constexpr basic_string_span(pointer firstElem, pointer lastElem) : span_(firstElem, lastElem) {}
 
     // From static arrays - if 0-terminated, remove 0 from the view
-
-    // from static arrays and string literals
+    // All other containers allow 0s within the length, so we do not remove them
     template <size_t N>
-    constexpr basic_string_span(value_type (&arr)[N]) noexcept : span_(remove_z(arr))
+    constexpr basic_string_span(element_type (&arr)[N]) : span_(remove_z(arr))
+    {
+    }
+    
+    template <size_t N, class ArrayElementType = std::remove_const_t<element_type>>
+    constexpr basic_string_span(std::array<ArrayElementType, N>& arr) noexcept
+    : span_(arr) {}
+
+    template <size_t N, class ArrayElementType = std::remove_const_t<element_type>>
+    constexpr basic_string_span(const std::array<ArrayElementType, N>& arr) noexcept
+    : span_(arr) {}
+
+    // Container signature should work for basic_string after C++17 version exists
+    template <class Traits, class Allocator>
+    constexpr basic_string_span(std::basic_string<element_type, Traits, Allocator>& str)
+    : span_(&str[0], str.length())
     {
     }
 
-    // Those allow 0s within the length, so we do not remove them
-
-    // from raw data and length
-    constexpr basic_string_span(pointer ptr, size_type length) noexcept : span_(ptr, length) {}
-
-    // from string
-    constexpr basic_string_span(std::string& s) noexcept
-        : span_(const_cast<pointer>(s.data()), narrow_cast<std::ptrdiff_t>(s.length()))
+    template <class Traits, class Allocator>
+    constexpr basic_string_span(const std::basic_string<element_type, Traits, Allocator>& str)
+    : span_(&str[0], str.length())
     {
     }
 
-    // from containers. Containers must have .size() and .data() function signatures
-    template <typename Cont,
-              typename = std::enable_if_t<
-                  !details::is_basic_string_span<Cont>::value && !details::is_span<Cont>::value &&
-                  std::is_convertible<Cont::pointer, pointer>::value &&
-                  std::is_convertible<Cont::pointer, decltype(std::declval<Cont>().data())>::value>>
-    constexpr basic_string_span(Cont& cont) : span_(cont.data(), cont.size())
+    // from containers. Containers must have a pointer type and data() function signatures
+    template <class Container,
+              class = std::enable_if_t<
+                  !details::is_basic_string_span<Container>::value &&
+                  !details::is_span<Container>::value &&
+                  std::is_convertible<typename Container::pointer, pointer>::value &&
+                  std::is_convertible<typename Container::pointer, decltype(std::declval<Container>().data())>::value>>
+    constexpr basic_string_span(Container& cont) : span_(cont)
     {
     }
 
-    // disallow creation from temporary containers and strings
-    template <typename Cont,
-              typename = std::enable_if_t<
-                  !details::is_basic_string_span<Cont>::value && !details::is_span<Cont>::value &&
-                  std::is_convertible<Cont::pointer, pointer>::value &&
-                  std::is_convertible<Cont::pointer, decltype(std::declval<Cont>().data())>::value>>
-    constexpr basic_string_span(const Cont& cont) : span_(cont.data(), cont.size())
+    template <class Container,
+              class = std::enable_if_t<
+                  !details::is_basic_string_span<Container>::value &&
+                  !details::is_span<Container>::value &&
+                  std::is_convertible<typename Container::pointer, pointer>::value &&
+                  std::is_convertible<typename Container::pointer, decltype(std::declval<Container>().data())>::value>>
+    constexpr basic_string_span(const Container& cont) : span_(cont)
     {
     }
 
@@ -326,109 +333,97 @@ public:
     template <typename OtherValueType, std::ptrdiff_t OtherExtent,
               typename Dummy = std::enable_if_t<
                   std::is_convertible<span<OtherValueType, OtherExtent>, impl_type>::value>>
-    constexpr basic_string_span(span<OtherValueType, OtherExtent> other) noexcept : span_(other)
+    constexpr basic_string_span(const span<OtherValueType, OtherExtent>& other) : span_(other)
     {
     }
 #else
     // from span
-    constexpr basic_string_span(span<value_type, Extent> other) noexcept : span_(other) {}
+    constexpr basic_string_span(span<element_type, Extent> other) : span_(other) {}
 
     template <typename = std::enable_if_t<
-                  !std::is_same<std::remove_const_t<value_type>, value_type>::value>>
-    constexpr basic_string_span(span<std::remove_const_t<value_type>, Extent> other) noexcept
+                  !std::is_same<std::remove_const_t<element_type>, value_type>::value>>
+    constexpr basic_string_span(const span<std::remove_const_t<element_type>, Extent>& other) 
         : span_(other)
     {
     }
 #endif
 
     // from string_span
-    template <typename OtherValueType, std::ptrdiff_t OtherExtent,
-              typename = std::enable_if_t<std::is_convertible<
-                  basic_string_span<OtherValueType, OtherExtent>::impl_type, impl_type>::value>>
-    constexpr basic_string_span(basic_string_span<OtherValueType, OtherExtent> other) noexcept
+    template <class OtherValueType, std::ptrdiff_t OtherExtent,
+              class = std::enable_if_t<std::is_convertible<
+                  typename basic_string_span<OtherValueType, OtherExtent>::impl_type, impl_type>::value>>
+    constexpr basic_string_span(basic_string_span<OtherValueType, OtherExtent> other) 
         : span_(other.data(), other.length())
     {
     }
 
-    constexpr bool empty() const noexcept { return length() == 0; }
-
     // first Count elements
-    template <size_type Count>
-    constexpr basic_string_span<value_type, Count> first() const noexcept
+    template <index_type Count>
+    constexpr basic_string_span<element_type, Count> first() const 
     {
         return {span_.template first<Count>()};
     }
 
-    constexpr basic_string_span<value_type, dynamic_extent> first(size_type count) const noexcept
+    constexpr basic_string_span<index_type, dynamic_extent> first(index_type count) const 
     {
         return {span_.first(count)};
     }
 
     // last Count elements
-    template <size_type Count>
-    constexpr basic_string_span<value_type, Count> last() const noexcept
+    template <index_type Count>
+    constexpr basic_string_span<index_type, Count> last() const 
     {
         return {span_.template last<Count>()};
     }
 
-    constexpr basic_string_span<value_type, dynamic_extent> last(size_type count) const noexcept
+    constexpr basic_string_span<element_type, dynamic_extent> last(index_type count) const 
     {
         return {span_.last(count)};
     }
 
-    // create a subview of Count elements starting from Offset
-    template <size_type Offset, size_type Count>
-    constexpr basic_string_span<value_type, Count> subspan() const noexcept
+    template <index_type Offset, index_type Count>
+    constexpr basic_string_span<element_type, Count> subspan() const 
     {
         return {span_.template subspan<Offset, Count>()};
     }
 
-    constexpr basic_string_span<value_type, dynamic_extent>
-    subspan(size_type offset, size_type count = dynamic_extent) const noexcept
+    constexpr basic_string_span<element_type, dynamic_extent>
+    subspan(index_type offset, index_type count = dynamic_extent) const 
     {
         return {span_.subspan(offset, count)};
     }
 
-    constexpr reference operator[](size_type idx) const noexcept { return span_[idx]; }
+    constexpr reference operator[](index_type idx) const { return span_[idx]; }
+    constexpr reference operator()(index_type idx) const { return span_[idx]; }
 
-    constexpr pointer data() const noexcept { return span_.data(); }
+    constexpr pointer data() const { return span_.data(); }
 
-    // length of the span in elements
-    constexpr size_type length() const noexcept { return span_.size(); }
-
-    // length of the span in elements
-    constexpr size_type size() const noexcept { return span_.size(); }
-
-    // length of the span in bytes
-    constexpr size_type size_bytes() const noexcept { return span_.size_bytes(); }
-
-    // length of the span in bytes
-    constexpr size_type length_bytes() const noexcept { return span_.length_bytes(); }
+    constexpr index_type length() const noexcept { return span_.size(); }
+    constexpr index_type size() const noexcept { return span_.size(); }
+    constexpr index_type size_bytes() const noexcept { return span_.size_bytes(); }
+    constexpr index_type length_bytes() const noexcept { return span_.length_bytes(); }
+    constexpr bool empty() const noexcept { return size() == 0; }
 
     constexpr iterator begin() const noexcept { return span_.begin(); }
-
     constexpr iterator end() const noexcept { return span_.end(); }
-
+    
     constexpr const_iterator cbegin() const noexcept { return span_.cbegin(); }
-
     constexpr const_iterator cend() const noexcept { return span_.cend(); }
-
+    
     constexpr reverse_iterator rbegin() const noexcept { return span_.rbegin(); }
-
     constexpr reverse_iterator rend() const noexcept { return span_.rend(); }
 
     constexpr const_reverse_iterator crbegin() const noexcept { return span_.crbegin(); }
-
     constexpr const_reverse_iterator crend() const noexcept { return span_.crend(); }
 
 private:
-    static impl_type remove_z(pointer const& sz, std::ptrdiff_t max) noexcept
+    static impl_type remove_z(pointer const& sz, std::ptrdiff_t max) 
     {
-        return {sz, details::length_func<value_type>()(sz, max)};
+        return {sz, details::length_func<element_type>()(sz, max)};
     }
 
     template <size_t N>
-    static impl_type remove_z(value_type (&sz)[N]) noexcept
+    static impl_type remove_z(element_type (&sz)[N]) 
     {
         return remove_z(&sz[0], narrow_cast<std::ptrdiff_t>(N));
     }
@@ -453,7 +448,7 @@ using cwstring_span = basic_string_span<const wchar_t, Extent>;
 //
 #ifndef GSL_MSVC_HAS_TYPE_DEDUCTION_BUG
 
-template <typename CharT, ptrdiff_t Extent>
+template <typename CharT, std::ptrdiff_t Extent>
 std::basic_string<typename std::remove_const<CharT>::type>
 to_string(basic_string_span<CharT, Extent> view)
 {
@@ -559,15 +554,13 @@ using czstring_span = basic_zstring_span<const char, Max>;
 template <std::ptrdiff_t Max = dynamic_extent>
 using cwzstring_span = basic_zstring_span<const wchar_t, Max>;
 
-} // namespace GSL
-
 // operator ==
-template <typename CharT, std::ptrdiff_t Extent = gsl::dynamic_extent, typename T,
-          typename = std::enable_if_t<std::is_convertible<
-              T, gsl::basic_string_span<std::add_const_t<CharT>, Extent>>::value>>
-bool operator==(gsl::basic_string_span<CharT, Extent> one, const T& other) noexcept
+template <class CharT, std::ptrdiff_t Extent, class T,
+          class = std::enable_if_t<details::is_basic_string_span<T>::value || std::is_convertible<
+              T, gsl::basic_string_span<std::add_const_t<CharT>>>::value>>
+bool operator==(const gsl::basic_string_span<CharT, Extent>& one, const T& other) noexcept
 {
-    gsl::basic_string_span<std::add_const_t<CharT>, Extent> tmp(other);
+    gsl::basic_string_span<std::add_const_t<CharT>> tmp(other);
 #ifdef GSL_MSVC_NO_CPP14_STD_EQUAL
     return (one.size() == tmp.size()) && std::equal(one.begin(), one.end(), tmp.begin());
 #else
@@ -576,13 +569,13 @@ bool operator==(gsl::basic_string_span<CharT, Extent> one, const T& other) noexc
 }
 
 template <
-    typename CharT, std::ptrdiff_t Extent = gsl::dynamic_extent, typename T,
-    typename Dummy = std::enable_if_t<
-        std::is_convertible<T, gsl::basic_string_span<std::add_const_t<CharT>, Extent>>::value &&
-        !gsl::details::is_basic_string_span<T>::value>>
-bool operator==(const T& one, gsl::basic_string_span<CharT, Extent> other) noexcept
+    class CharT, std::ptrdiff_t Extent, class T,
+    class = std::enable_if_t<!details::is_basic_string_span<T>::value && 
+        std::is_convertible<T, gsl::basic_string_span<std::add_const_t<CharT>>>::value 
+        >>
+bool operator==(const T& one, const gsl::basic_string_span<CharT, Extent>& other) noexcept
 {
-    gsl::basic_string_span<std::add_const_t<CharT>, Extent> tmp(one);
+    gsl::basic_string_span<std::add_const_t<CharT>> tmp(one);
 #ifdef GSL_MSVC_NO_CPP14_STD_EQUAL
     return (tmp.size() == other.size()) && std::equal(tmp.begin(), tmp.end(), other.begin());
 #else
@@ -590,40 +583,6 @@ bool operator==(const T& one, gsl::basic_string_span<CharT, Extent> other) noexc
 #endif
 }
 
-#ifndef _MSC_VER
-
-// VS treats temp and const containers as convertible to basic_string_span,
-// so the cases below are already covered by the previous operators
-
-template <
-    typename CharT, std::ptrdiff_t Extent = gsl::dynamic_extent, typename T,
-    typename DataType = typename T::value_type,
-    typename Dummy = std::enable_if_t<
-        !gsl::details::is_span<T>::value && !gsl::details::is_basic_string_span<T>::value &&
-        std::is_convertible<DataType*, CharT*>::value &&
-        std::is_same<std::decay_t<decltype(std::declval<T>().size(), *std::declval<T>().data())>,
-                     DataType>::value>>
-bool operator==(gsl::basic_string_span<CharT, Extent> one, const T& other) noexcept
-{
-    gsl::basic_string_span<std::add_const_t<CharT>, Extent> tmp(other);
-    return std::equal(one.begin(), one.end(), tmp.begin(), tmp.end());
-}
-
-template <
-    typename CharT, std::ptrdiff_t Extent = gsl::dynamic_extent, typename T,
-    typename DataType = typename T::value_type,
-    typename Dummy = std::enable_if_t<
-        !gsl::details::is_span<T>::value && !gsl::details::is_basic_string_span<T>::value &&
-        std::is_convertible<DataType*, CharT*>::value &&
-        std::is_same<std::decay_t<decltype(std::declval<T>().size(), *std::declval<T>().data())>,
-                     DataType>::value>>
-bool operator==(const T& one, gsl::basic_string_span<CharT, Extent> other) noexcept
-{
-    gsl::basic_string_span<std::add_const_t<CharT>, Extent> tmp(one);
-    return std::equal(tmp.begin(), tmp.end(), other.begin(), other.end());
-}
-#endif
-
 // operator !=
 template <typename CharT, std::ptrdiff_t Extent = gsl::dynamic_extent, typename T,
           typename = std::enable_if_t<std::is_convertible<
@@ -643,38 +602,6 @@ bool operator!=(const T& one, gsl::basic_string_span<CharT, Extent> other) noexc
     return !(one == other);
 }
 
-#ifndef _MSC_VER
-
-// VS treats temp and const containers as convertible to basic_string_span,
-// so the cases below are already covered by the previous operators
-
-template <
-    typename CharT, std::ptrdiff_t Extent = gsl::dynamic_extent, typename T,
-    typename DataType = typename T::value_type,
-    typename Dummy = std::enable_if_t<
-        !gsl::details::is_span<T>::value && !gsl::details::is_basic_string_span<T>::value &&
-        std::is_convertible<DataType*, CharT*>::value &&
-        std::is_same<std::decay_t<decltype(std::declval<T>().size(), *std::declval<T>().data())>,
-                     DataType>::value>>
-bool operator!=(gsl::basic_string_span<CharT, Extent> one, const T& other) noexcept
-{
-    return !(one == other);
-}
-
-template <
-    typename CharT, std::ptrdiff_t Extent = gsl::dynamic_extent, typename T,
-    typename DataType = typename T::value_type,
-    typename Dummy = std::enable_if_t<
-        !gsl::details::is_span<T>::value && !gsl::details::is_basic_string_span<T>::value &&
-        std::is_convertible<DataType*, CharT*>::value &&
-        std::is_same<std::decay_t<decltype(std::declval<T>().size(), *std::declval<T>().data())>,
-                     DataType>::value>>
-bool operator!=(const T& one, gsl::basic_string_span<CharT, Extent> other) noexcept
-{
-    return !(one == other);
-}
-#endif
-
 // operator<
 template <typename CharT, std::ptrdiff_t Extent = gsl::dynamic_extent, typename T,
           typename = std::enable_if_t<std::is_convertible<
@@ -882,6 +809,7 @@ bool operator>=(const T& one, gsl::basic_string_span<CharT, Extent> other) noexc
     return !(one < other);
 }
 #endif
+} // namespace GSL
 
 #ifdef _MSC_VER
 
diff --git a/tests/string_span_tests.cpp b/tests/string_span_tests.cpp
index f380be3..876886a 100644
--- a/tests/string_span_tests.cpp
+++ b/tests/string_span_tests.cpp
@@ -40,14 +40,14 @@ SUITE(string_span_tests)
     {
         std::string s = "Hello there world";
         cstring_span<> v = s;
-        CHECK(v.length() == static_cast<cstring_span<>::size_type>(s.length()));
+        CHECK(v.length() == static_cast<cstring_span<>::index_type>(s.length()));
     }
 
     TEST(TestConstructFromStdVector)
     {
         std::vector<char> vec(5, 'h');
         string_span<> v {vec};
-        CHECK(v.length() == static_cast<string_span<>::size_type>(vec.size()));
+        CHECK(v.length() == static_cast<string_span<>::index_type>(vec.size()));
     }
 
     TEST(TestStackArrayConstruction)
@@ -109,7 +109,7 @@ SUITE(string_span_tests)
         char stack_string[] = "Hello";
         cstring_span<> v = ensure_z(stack_string);
         auto s2 = gsl::to_string(v);
-        CHECK(static_cast<cstring_span<>::size_type>(s2.length()) == v.length());
+        CHECK(static_cast<cstring_span<>::index_type>(s2.length()) == v.length());
         CHECK(s2.length() == 5);
     }