From 201dd99b6494564c9e8990d0a5c32b23431128d4 Mon Sep 17 00:00:00 2001
From: Kirk Shoop <kirk.shoop@microsoft.com>
Date: Mon, 10 Apr 2017 16:03:58 -0700
Subject: Kirkshoop/filterimprovement (#371)

* filter: mutable predicate and value forwarding

* add as_const to protect predicate from stealing
---
 Rx/v2/src/rxcpp/operators/rx-filter.hpp | 11 ++++++----
 Rx/v2/src/rxcpp/rx-util.hpp             | 38 +++++++++++++++++++++------------
 2 files changed, 31 insertions(+), 18 deletions(-)

diff --git a/Rx/v2/src/rxcpp/operators/rx-filter.hpp b/Rx/v2/src/rxcpp/operators/rx-filter.hpp
index ef6e133..86ce649 100644
--- a/Rx/v2/src/rxcpp/operators/rx-filter.hpp
+++ b/Rx/v2/src/rxcpp/operators/rx-filter.hpp
@@ -58,22 +58,25 @@ struct filter
         typedef rxu::decay_t<Subscriber> dest_type;
         typedef observer<value_type, this_type> observer_type;
         dest_type dest;
-        test_type test;
+        mutable test_type test;
 
         filter_observer(dest_type d, test_type t)
             : dest(std::move(d))
             , test(std::move(t))
         {
         }
-        void on_next(source_value_type v) const {
+
+        template <class Value>
+        void on_next(Value&& v) const {
             auto filtered = on_exception([&](){
-                return !this->test(v);},
+                    return !this->test(rxu::as_const(v));
+                },
                 dest);
             if (filtered.empty()) {
                 return;
             }
             if (!filtered.get()) {
-                dest.on_next(v);
+                dest.on_next(std::forward<Value>(v));
             }
         }
         void on_error(std::exception_ptr e) const {
diff --git a/Rx/v2/src/rxcpp/rx-util.hpp b/Rx/v2/src/rxcpp/rx-util.hpp
index 77e0d3f..9c6534f 100644
--- a/Rx/v2/src/rxcpp/rx-util.hpp
+++ b/Rx/v2/src/rxcpp/rx-util.hpp
@@ -51,6 +51,16 @@ typename std::enable_if<!std::is_array<T0>::value && std::is_pod<T0>::value, std
     return to_vector({t0, tn...});
 }
 
+// lifted from https://github.com/ericniebler/range-v3/blob/630fc70baa07cbfd222f329e44a3122ab64ce364/include/range/v3/range_fwd.hpp
+// removed constexpr & noexcept to support older VC compilers
+template<typename T>
+/*constexpr*/ T const &as_const(T & t) /*noexcept*/
+{
+    return t;
+}
+template<typename T>
+void as_const(T const &&) = delete;
+
 template<class T, T... ValueN>
 struct values {};
 
@@ -157,7 +167,7 @@ using types_checked_t = typename types_checked_from<TN...>::type;
 template<class Types, class =types_checked>
 struct expand_value_types { struct type; };
 template<class... TN>
-struct expand_value_types<types<TN...>, types_checked_t<typename std::decay<TN>::type::value_type...>> 
+struct expand_value_types<types<TN...>, types_checked_t<typename std::decay<TN>::type::value_type...>>
 {
     using type = types<typename std::decay<TN>::type::value_type...>;
 };
@@ -517,7 +527,7 @@ inline std::string what(std::exception_ptr ep) {
     }
     return std::string();
 }
-                
+
 namespace detail {
 
 template <class T>
@@ -754,7 +764,7 @@ struct is_string : std::false_type {
 };
 
 template <typename T>
-struct is_string<T, 
+struct is_string<T,
     typename types_checked_from<
         typename T::value_type,
         typename T::traits_type,
@@ -796,32 +806,32 @@ namespace rxu=util;
 
 
 //
-// due to an noisy static_assert issue in more than one std lib impl, 
-// rxcpp maintains a whitelist filter for the types that are allowed 
+// due to an noisy static_assert issue in more than one std lib impl,
+// rxcpp maintains a whitelist filter for the types that are allowed
 // to be hashed. this allows is_hashable<T> to work.
 //
 // NOTE: this should eventually be removed!
 //
-template <class T, typename = void> 
+template <class T, typename = void>
 struct filtered_hash;
 
 #if RXCPP_HASH_ENUM
-template <class T> 
+template <class T>
 struct filtered_hash<T, typename std::enable_if<std::is_enum<T>::value>::type> : std::hash<T> {
 };
 #elif RXCPP_HASH_ENUM_UNDERLYING
-template <class T> 
+template <class T>
 struct filtered_hash<T, typename std::enable_if<std::is_enum<T>::value>::type> : std::hash<typename std::underlying_type<T>::type> {
 };
 #endif
 
-template <class T> 
+template <class T>
 struct filtered_hash<T, typename std::enable_if<std::is_integral<T>::value>::type> : std::hash<T> {
 };
-template <class T> 
+template <class T>
 struct filtered_hash<T, typename std::enable_if<std::is_pointer<T>::value>::type> : std::hash<T> {
 };
-template <class T> 
+template <class T>
 struct filtered_hash<T, typename std::enable_if<rxu::is_string<T>::value>::type> : std::hash<T> {
 };
 template <class T>
@@ -850,10 +860,10 @@ struct is_hashable
     : std::false_type {};
 
 template<typename T>
-struct is_hashable<T, 
+struct is_hashable<T,
     typename rxu::types_checked_from<
-        typename filtered_hash<T>::result_type, 
-        typename filtered_hash<T>::argument_type, 
+        typename filtered_hash<T>::result_type,
+        typename filtered_hash<T>::argument_type,
         typename std::result_of<filtered_hash<T>(T)>::type>::type>
     : std::true_type {};
 
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