From 3a7c8cb71501eae55099c494c78e90b305deeadd Mon Sep 17 00:00:00 2001
From: Hans Dembinski <hans.dembinski@gmail.com>
Date: Sun, 11 Mar 2018 20:51:03 +0100
Subject: [PATCH] always return matching types in bin_views

---
 include/boost/histogram/axis/axis.hpp              |  2 +-
 include/boost/histogram/axis/bin_view.hpp          |  6 +++---
 include/boost/histogram/axis/ostream_operators.hpp |  2 +-
 include/boost/histogram/detail/meta.hpp            | 11 +++++++++++
 src/python/axis.cpp                                |  5 +----
 5 files changed, 17 insertions(+), 9 deletions(-)

diff --git a/include/boost/histogram/axis/axis.hpp b/include/boost/histogram/axis/axis.hpp
index 87f7be79..7aec7abd 100644
--- a/include/boost/histogram/axis/axis.hpp
+++ b/include/boost/histogram/axis/axis.hpp
@@ -552,7 +552,7 @@ public:
   }
 
   /// Returns the value for the bin index (performs a range check).
-  inline value_type value(int idx) const {
+  inline const value_type& value(int idx) const {
     auto it = map_->right.find(idx);
     if (it == map_->right.end())
       throw std::out_of_range("category index out of range");
diff --git a/include/boost/histogram/axis/bin_view.hpp b/include/boost/histogram/axis/bin_view.hpp
index ce9b6e3b..5d672d0d 100644
--- a/include/boost/histogram/axis/bin_view.hpp
+++ b/include/boost/histogram/axis/bin_view.hpp
@@ -26,8 +26,8 @@ public:
 
   int idx() const noexcept { return idx_; }
 
-  typename Axis::value_type lower() const noexcept { return axis_.lower(idx_); }
-  typename Axis::value_type upper() const noexcept { return axis_.lower(idx_+1); }
+  auto lower() const noexcept -> decltype(std::declval<Axis&>().lower(0)) { return axis_.lower(idx_); }
+  auto upper() const noexcept -> decltype(std::declval<Axis&>().lower(0)) { return axis_.lower(idx_+1); }
   typename Axis::value_type width() const noexcept { return upper() - lower(); }
 
   bool operator==(const interval_view &rhs) const noexcept {
@@ -56,7 +56,7 @@ public:
 
   int idx() const noexcept { return idx_; }
 
-  typename Axis::value_type value() const {
+  auto value() const -> decltype(std::declval<Axis&>().value(0)) {
     return axis_.value(idx_);
   }
 
diff --git a/include/boost/histogram/axis/ostream_operators.hpp b/include/boost/histogram/axis/ostream_operators.hpp
index 6be6d7dc..73b632de 100644
--- a/include/boost/histogram/axis/ostream_operators.hpp
+++ b/include/boost/histogram/axis/ostream_operators.hpp
@@ -137,7 +137,7 @@ inline std::ostream &operator<<(std::ostream &os,
                                 const category<std::string> &a) {
   os << "category(";
   for (int i = 0; i < a.size(); ++i) {
-    ::boost::histogram::detail::escape(os, a[i].value());
+    ::boost::histogram::detail::escape(os, a.value(i));
     os << (i == (a.size() - 1) ? "" : ", ");
   }
   if (!a.label().empty()) {
diff --git a/include/boost/histogram/detail/meta.hpp b/include/boost/histogram/detail/meta.hpp
index 691e325b..be75a7d9 100644
--- a/include/boost/histogram/detail/meta.hpp
+++ b/include/boost/histogram/detail/meta.hpp
@@ -47,6 +47,17 @@ template <typename T> struct has_variance_support {
 template <typename T>
 using has_variance_support_t = typename has_variance_support<T>::type;
 
+template <typename T> struct has_method_lower {
+  template <typename U, typename = decltype(std::declval<U &>().lower(0))>
+  struct SFINAE {};
+  template <typename U> static std::true_type Test(SFINAE<U> *);
+  template <typename U> static std::false_type Test(...);
+  using type = decltype(Test<T>(nullptr));
+};
+
+template <typename T>
+using has_method_lower_t = typename has_method_lower<T>::type;
+
 template <typename T,
           typename = decltype(*std::declval<T &>(), ++std::declval<T &>())>
 struct is_iterator {};
diff --git a/src/python/axis.cpp b/src/python/axis.cpp
index 736e5d23..9a0f8c50 100644
--- a/src/python/axis.cpp
+++ b/src/python/axis.cpp
@@ -153,10 +153,7 @@ template <typename A> bp::object axis_getitem(const A &a, int i) {
     PyErr_SetString(PyExc_IndexError, "index out of bounds");
     bp::throw_error_already_set();
   }
-  return axis_getitem_impl(std::is_same<
-      typename A::bin_type, bha::interval_view<A>
-    >(), a, i);
-  return bp::object();
+  return axis_getitem_impl(bh::detail::has_method_lower_t<A>(), a, i);
 }
 
 template <typename T> void axis_set_label(T& t, bp::str s) {