From 4e7febfa2ca9a3c95470b8d78eade9d2fdfda5e3 Mon Sep 17 00:00:00 2001
From: Hans Dembinski <hans.dembinski@gmail.com>
Date: Sun, 2 Jun 2019 21:35:27 +0200
Subject: [PATCH] get_if may not be called if there is more than one match

---
 include/boost/histogram/detail/variant.hpp | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/include/boost/histogram/detail/variant.hpp b/include/boost/histogram/detail/variant.hpp
index dba9f219..36eef927 100644
--- a/include/boost/histogram/detail/variant.hpp
+++ b/include/boost/histogram/detail/variant.hpp
@@ -130,12 +130,14 @@ public:
 
   template <class T>
   T* get_if() noexcept {
+    static_assert(mp11::mp_count<variant, T>::value <= 1, "no unique match for T");
     constexpr auto i = mp11::mp_find<variant, T>{};
     return i == index_ ? &ref<T>() : nullptr;
   }
 
   template <class T>
   const T* get_if() const noexcept {
+    static_assert(mp11::mp_count<variant, T>::value <= 1, "no unique match for T");
     constexpr auto i = mp11::mp_find<variant, T>{};
     return i == index_ ? &ref<T>() : nullptr;
   }