Fix EnableIfContainer with MSVC 2015.

MSVC 2015's SFINAE implementation is broken. In particular, it seems not
to bother expanding EnableIfContainer unless we force it to by writing
::type. That means we need to use std::enable_if rather than
enable_if_t, even though it's quite wordy.

Change-Id: Ic643ab8a956991bb14af07832be80988f7735428
Reviewed-on: https://boringssl-review.googlesource.com/20764
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

include/openssl/span.h

@@ -92,13 +92,15 @@ class SpanBase {
 template <typename T>
 class Span : private internal::SpanBase<const T> {
  private:
-  template <bool B, class V = void>
-  using enable_if_t = typename std::enable_if<B, V>::type;
-
   // Heuristically test whether C is a container type that can be converted into
   // a Span by checking for data() and size() member functions.
+  //
+  // TODO(davidben): Switch everything to std::enable_if_t when we remove
+  // support for MSVC 2015. Although we could write our own enable_if_t and MSVC
+  // 2015 has std::enable_if_t anyway, MSVC 2015's SFINAE implementation is
+  // problematic and does not work below unless we write the ::type at use.
   template <typename C>
-  using EnableIfContainer = enable_if_t<
+  using EnableIfContainer = std::enable_if<
       std::is_convertible<decltype(std::declval<C>().data()), T *>::value &&
       std::is_integral<decltype(std::declval<C>().size())>::value>;
 
@@ -109,12 +111,14 @@ class Span : private internal::SpanBase<const T> {
   template <size_t N>
   constexpr Span(T (&array)[N]) : Span(array, N) {}
 
-  template <typename C, typename = EnableIfContainer<C>,
-            typename = enable_if_t<std::is_const<T>::value, C>>
+  template <
+      typename C, typename = typename EnableIfContainer<C>::type,
+      typename = typename std::enable_if<std::is_const<T>::value, C>::type>
   Span(const C &container) : data_(container.data()), size_(container.size()) {}
 
-  template <typename C, typename = EnableIfContainer<C>,
-            typename = enable_if_t<!std::is_const<T>::value, C>>
+  template <
+      typename C, typename = typename EnableIfContainer<C>::type,
+      typename = typename std::enable_if<!std::is_const<T>::value, C>::type>
   explicit Span(C &container)
       : data_(container.data()), size_(container.size()) {}