From c5c251fdbf1af92567c8a6696dacdc6af8fe281d Mon Sep 17 00:00:00 2001
From: Hans Dembinski <HDembinski@users.noreply.github.com>
Date: Tue, 24 Sep 2019 22:41:49 +0100
Subject: [PATCH] return copy instead of reference for scalar types (#218)

---
 include/boost/histogram/axis/category.hpp |  3 ++-
 test/axis_category_test.cpp               | 10 ++++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/include/boost/histogram/axis/category.hpp b/include/boost/histogram/axis/category.hpp
index 309aae0d..14a2bac3 100644
--- a/include/boost/histogram/axis/category.hpp
+++ b/include/boost/histogram/axis/category.hpp
@@ -128,7 +128,8 @@ public:
 
   /// Return value for index argument.
   /// Throws `std::out_of_range` if the index is out of bounds.
-  decltype(auto) value(index_type idx) const {
+  std::conditional_t<std::is_scalar<value_type>::value, value_type, const value_type&>
+  value(index_type idx) const {
     if (idx < 0 || idx >= size())
       BOOST_THROW_EXCEPTION(std::out_of_range("category index out of range"));
     return vec_meta_.first()[idx];
diff --git a/test/axis_category_test.cpp b/test/axis_category_test.cpp
index 397554de..ed7c4f75 100644
--- a/test/axis_category_test.cpp
+++ b/test/axis_category_test.cpp
@@ -5,6 +5,7 @@
 // or copy at http://www.boost.org/LICENSE_1_0.txt)
 
 #include <boost/core/lightweight_test.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
 #include <boost/histogram/axis/category.hpp>
 #include <boost/histogram/axis/ostream.hpp>
 #include <boost/histogram/detail/cat.hpp>
@@ -64,6 +65,15 @@ int main() {
     BOOST_TEST_NE(c, d);
     d = std::move(c);
     BOOST_TEST_EQ(d, a);
+
+    enum class Foo { foo };
+
+    BOOST_TEST_TRAIT_SAME(decltype(std::declval<axis::category<std::string>>().value(0)),
+                          const std::string&);
+    BOOST_TEST_TRAIT_SAME(decltype(std::declval<axis::category<const char*>>().value(0)),
+                          const char*);
+    BOOST_TEST_TRAIT_SAME(decltype(std::declval<axis::category<Foo>>().value(0)), Foo);
+    BOOST_TEST_TRAIT_SAME(decltype(std::declval<axis::category<int>>().value(0)), int);
   }
 
   // axis::category with growth