From 35c265f313624066493c3b4df665c7bc43246cd3 Mon Sep 17 00:00:00 2001
From: joaquintides <joaquin@tid.es>
Date: Mon, 10 Jul 2023 12:07:42 +0200
Subject: [PATCH] worked around C3517 error in VS2017 and older

---
 .../detail/foa/takes_arg_as_const_reference.hpp       | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/include/boost/unordered/detail/foa/takes_arg_as_const_reference.hpp b/include/boost/unordered/detail/foa/takes_arg_as_const_reference.hpp
index f8f446da..8db21c92 100644
--- a/include/boost/unordered/detail/foa/takes_arg_as_const_reference.hpp
+++ b/include/boost/unordered/detail/foa/takes_arg_as_const_reference.hpp
@@ -142,13 +142,20 @@ template<typename F,typename /*Arg*/,typename=void>
 struct takes_arg_as_const_reference:
   has_const_reference_arg<remove_noexcept_t<F>>{};
 
+/* VS2017 and older issue a C3517 error when trying to obtain the type of
+ * an instantiation of a template function with deduced return type if
+ * the instantiation has not been evaluated before. Passing through this
+ * function solves the issue
+ */
+template<typename T> constexpr T force_evaluation(T);
+
 template<typename F,typename Arg>
 struct takes_arg_as_const_reference<
   F,Arg,
-  boost::void_t<decltype(&F::template operator()<Arg>)>
+  boost::void_t<decltype(force_evaluation(&F::template operator()<Arg>))>
 >:
 takes_arg_as_const_reference<
-  decltype(&F::template operator()<Arg>),Arg
+  decltype(force_evaluation(&F::template operator()<Arg>)),Arg
 >{};
 
 template<typename F,typename Arg>