message: Enable constexpr tests for Optional on Windows now that we're using a version of Visual Studio that has sufficient constexpr support.
diff: @@ -281,19 +281,14 @@ TEST(Optional_ConstExpr)
  CHECK_EQUAL(bool(c), true);
  constexpr int d = *c;
  CHECK_EQUAL(1, d);
  -// FIXME: Visual Studio 2015's constexpr support isn't sufficient to allow this to compile.
  -#ifndef _WIN32
  constexpr bool e = bool(Optional<int>{1});
  CHECK_EQUAL(true, e);
  constexpr bool f = bool(Optional<int>{none});
  CHECK_EQUAL(false, f);
  -#endif
  constexpr int g = b.value_or(1234);
  CHECK_EQUAL(1234, g);
  }
  -// FIXME: Visual Studio 2015's constexpr support isn't sufficient to allow Optional<T&> to compile.
  -#ifndef _WIN32
  TEST(Optional_ReferenceConstExpr)
  {
  // Should compile:
  @@ -308,7 +303,6 @@ TEST(Optional_ReferenceConstExpr)
  constexpr bool f = bool(Optional<const int&>{none});
  CHECK_EQUAL(false, f);
  }
  -#endif
  // Disabled for compliance with std::optional
  // TEST(Optional_VoidIsEquivalentToBool)
  