Workaround for py::dict() constructor on MSVC

MSVC fails to compile if the constructor is defined out-of-line.
The error states that it cannot deduce the type of the default template
parameter which is used for SFINAE.

include/pybind11/cast.h

@@ -1134,10 +1134,6 @@ template <return_value_policy policy,
     return operator()<policy>(std::forward<Args>(args)...);
 }
 
-template <typename... Args, typename /*SFINAE*/>
-dict::dict(Args &&...args)
-    : dict(detail::unpacking_collector<>(std::forward<Args>(args)...).kwargs()) { }
-
 #define PYBIND11_MAKE_OPAQUE(Type) \
     namespace pybind11 { namespace detail { \
         template<> class type_caster<Type> : public type_caster_base<Type> { }; \

include/pybind11/common.h

@@ -358,6 +358,10 @@ template <template<typename> class P, typename T, typename... Ts>
 struct any_of_t<P, T, Ts...> : conditional_t<P<T>::value, std::true_type, any_of_t<P, Ts...>> { };
 #endif
 
+/// Defer the evaluation of type T until types Us are instantiated
+template <typename T, typename... /*Us*/> struct deferred_type { using type = T; };
+template <typename T, typename... Us> using deferred_t = typename deferred_type<T, Us...>::type;
+
 /// Ignore that a variable is unused in compiler warnings
 inline void ignore_unused(const int *) { }
 

include/pybind11/pytypes.h

@@ -259,6 +259,12 @@ template <typename T> using is_keyword_or_ds = bool_constant<
     is_keyword<T>::value || is_ds_unpacking<T>::value
 >;
 
+// Call argument collector forward declarations
+template <return_value_policy policy = return_value_policy::automatic_reference>
+class simple_collector;
+template <return_value_policy policy = return_value_policy::automatic_reference>
+class unpacking_collector;
+
 NAMESPACE_END(detail)
 
 #define PYBIND11_OBJECT_CVT(Name, Parent, CheckFun, CvtStmt) \
@@ -588,8 +594,11 @@ public:
         if (!m_ptr) pybind11_fail("Could not allocate dict object!");
     }
     template <typename... Args,
-              typename = detail::enable_if_t<detail::all_of_t<detail::is_keyword_or_ds, Args...>::value>>
-    dict(Args &&...args);
+              typename = detail::enable_if_t<detail::all_of_t<detail::is_keyword_or_ds, Args...>::value>,
+              // MSVC workaround: it can't compile an out-of-line definition, so defer the collector
+              typename collector = detail::deferred_t<detail::unpacking_collector<>, Args...>>
+    dict(Args &&...args) : dict(collector(std::forward<Args>(args)...).kwargs()) { }
+
     size_t size() const { return (size_t) PyDict_Size(m_ptr); }
     detail::dict_iterator begin() const { return (++detail::dict_iterator(*this, 0)); }
     detail::dict_iterator end() const { return detail::dict_iterator(); }