From 321a54728fcdefbed1b816cbf30323831cdc420d Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@pdimov.com>
Date: Mon, 20 Mar 2017 16:55:00 +0200
Subject: [PATCH] Add indirection in mp_quote to enable expansions of T... into
 a fixed parameter list

---
 include/boost/mp11/utility.hpp | 11 ++++++++++-
 test/mp_invoke.cpp             |  7 +++++++
 test/mp_quote.cpp              | 11 ++++++-----
 3 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/include/boost/mp11/utility.hpp b/include/boost/mp11/utility.hpp
index 2d7d2a1..78b5a9a 100644
--- a/include/boost/mp11/utility.hpp
+++ b/include/boost/mp11/utility.hpp
@@ -106,7 +106,16 @@ template<template<class...> class F, class... T> using mp_defer = mp_if<mp_valid
 // mp_quote
 template<template<class...> class F, class... T> struct mp_quote
 {
-    template<class... U> using fn = F<T..., U...>;
+private:
+
+    template<class... U> struct _fn { using type = F<T..., U...>; };
+
+public:
+
+    // the indirection through _fn works around the language inability
+    // to expand T.. to expand into a fixed parameter list of an alias template
+
+    template<class... U> using fn = typename _fn<U...>::type;
 };
 
 // mp_unquote
diff --git a/test/mp_invoke.cpp b/test/mp_invoke.cpp
index e97ca49..766e6bd 100644
--- a/test/mp_invoke.cpp
+++ b/test/mp_invoke.cpp
@@ -35,6 +35,11 @@ struct Q4
     template<class T1, class... T> using fn = T1;
 };
 
+struct Q5
+{
+    template<class T1, class T2> using fn = T2;
+};
+
 int main()
 {
     BOOST_TEST_TRAIT_TRUE((std::is_same<mp_invoke<Q1>, void>));
@@ -52,5 +57,7 @@ int main()
     BOOST_TEST_TRAIT_TRUE((std::is_same<mp_invoke<Q4, int>, int>));
     BOOST_TEST_TRAIT_TRUE((std::is_same<mp_invoke<Q4, int[], char[]>, int[]>));
 
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_invoke<Q5, int, float>, float>));
+
     return boost::report_errors();
 }
diff --git a/test/mp_quote.cpp b/test/mp_quote.cpp
index 3a338d3..29bb317 100644
--- a/test/mp_quote.cpp
+++ b/test/mp_quote.cpp
@@ -24,7 +24,7 @@ struct D1: B {};
 struct D2: B {};
 struct ND {};
 
-template<class... T> using is_base_of_t = typename std::is_base_of<T...>::type;
+template<class T, class U> using is_base_of_t = typename std::is_base_of<T, U>::type;
 
 int main()
 {
@@ -54,10 +54,11 @@ int main()
     {
         using Q = mp_quote<mp_identity_t>;
 
-        // using R1 = Y<Q::fn, void, char, int>;
-        // BOOST_TEST_TRAIT_TRUE((std::is_same<R1, X<void, char, int>>));
-        // 
-        // error: pack expansion used as argument for non-pack parameter of alias template
+#if defined( BOOST_MSVC ) && BOOST_WORKAROUND( BOOST_MSVC, <= 1800 )
+#else
+        using R1 = Y<Q::fn, void, char, int>;
+        BOOST_TEST_TRAIT_TRUE((std::is_same<R1, X<void, char, int>>));
+#endif
 
 #if defined( BOOST_MSVC ) && BOOST_WORKAROUND( BOOST_MSVC, <= 1910 && BOOST_MSVC >= 1900 )
 #else