Clean up cast operator invocations (#531)

This adds a `detail::cast_op<T>(caster)` function which handles the
rather verbose:

    caster.operator typename CasterType::template cast_op_type<T>()

which allows various places to use the shorter and clearer:

    cast_op<T>(caster)

instead of the full verbose cast operator invocation.

include/pybind11/cast.h

@@ -432,6 +432,16 @@ protected:
 template <typename type, typename SFINAE = void> class type_caster : public type_caster_base<type> { };
 template <typename type, typename SFINAE = void> class type_caster : public type_caster_base<type> { };
 template <typename type> using make_caster = type_caster<intrinsic_t<type>>;
 template <typename type> using make_caster = type_caster<intrinsic_t<type>>;
 
 
+// Shortcut for calling a caster's `cast_op_type` cast operator for casting a type_caster to a T
+template <typename T>
+auto cast_op(make_caster<T> &caster) -> decltype(caster.operator typename make_caster<T>::template cast_op_type<T>()) {
+    return caster.operator typename make_caster<T>::template cast_op_type<T>();
+}
+template <typename T>
+auto cast_op(make_caster<T> &&caster) -> decltype(caster.operator typename make_caster<T>::template cast_op_type<T>()) {
+    return cast_op<T>(caster);
+}
+
 template <typename type> class type_caster<std::reference_wrapper<type>> : public type_caster_base<type> {
 template <typename type> class type_caster<std::reference_wrapper<type>> : public type_caster_base<type> {
 public:
 public:
     static handle cast(const std::reference_wrapper<type> &src, return_value_policy policy, handle parent) {
     static handle cast(const std::reference_wrapper<type> &src, return_value_policy policy, handle parent) {
@@ -757,8 +767,7 @@ public:
     template <typename T> using cast_op_type = type;
     template <typename T> using cast_op_type = type;
 
 
     operator type() {
     operator type() {
-        return type(first.operator typename make_caster<T1>::template cast_op_type<T1>(),
-                    second.operator typename make_caster<T2>::template cast_op_type<T2>());
+        return type(cast_op<T1>(first), cast_op<T2>(second));
     }
     }
 protected:
 protected:
     make_caster<T1> first;
     make_caster<T1> first;
@@ -831,13 +840,11 @@ public:
 
 
 protected:
 protected:
     template <typename ReturnValue, typename Func, size_t ... Index> ReturnValue call(Func &&f, index_sequence<Index...>) {
     template <typename ReturnValue, typename Func, size_t ... Index> ReturnValue call(Func &&f, index_sequence<Index...>) {
-        return f(std::get<Index>(value)
-            .operator typename make_caster<Tuple>::template cast_op_type<Tuple>()...);
+        return f(cast_op<Tuple>(std::get<Index>(value))...);
     }
     }
 
 
     template <size_t ... Index> type cast(index_sequence<Index...>) {
     template <size_t ... Index> type cast(index_sequence<Index...>) {
-        return type(std::get<Index>(value)
-            .operator typename make_caster<Tuple>::template cast_op_type<Tuple>()...);
+        return type(cast_op<Tuple>(std::get<Index>(value))...);
     }
     }
 
 
     template <size_t ... Indices> bool load(handle src, bool convert, index_sequence<Indices...>) {
     template <size_t ... Indices> bool load(handle src, bool convert, index_sequence<Indices...>) {
@@ -1081,10 +1088,10 @@ NAMESPACE_END(detail)
 // pytype -> C++ type
 // pytype -> C++ type
 template <typename T, detail::enable_if_t<!detail::is_pyobject<T>::value, int> = 0>
 template <typename T, detail::enable_if_t<!detail::is_pyobject<T>::value, int> = 0>
 T cast(const handle &handle) {
 T cast(const handle &handle) {
-    static_assert(!detail::cast_is_temporary_value_reference<T>::value,
+    using namespace detail;
+    static_assert(!cast_is_temporary_value_reference<T>::value,
             "Unable to cast type to reference: value is local to type caster");
             "Unable to cast type to reference: value is local to type caster");
-    using type_caster = detail::make_caster<T>;
-    return detail::load_type<T>(handle).operator typename type_caster::template cast_op_type<T>();
+    return cast_op<T>(load_type<T>(handle));
 }
 }
 
 
 // pytype -> pytype (calls converting constructor)
 // pytype -> pytype (calls converting constructor)
@@ -1153,7 +1160,7 @@ template <typename ret_type> using overload_caster_t = conditional_t<
 // Trampoline use: for reference/pointer types to value-converted values, we do a value cast, then
 // Trampoline use: for reference/pointer types to value-converted values, we do a value cast, then
 // store the result in the given variable.  For other types, this is a no-op.
 // store the result in the given variable.  For other types, this is a no-op.
 template <typename T> enable_if_t<cast_is_temporary_value_reference<T>::value, T> cast_ref(object &&o, make_caster<T> &caster) {
 template <typename T> enable_if_t<cast_is_temporary_value_reference<T>::value, T> cast_ref(object &&o, make_caster<T> &caster) {
-    return load_type(caster, o).operator typename make_caster<T>::template cast_op_type<T>();
+    return cast_op<T>(load_type(caster, o));
 }
 }
 template <typename T> enable_if_t<!cast_is_temporary_value_reference<T>::value, T> cast_ref(object &&, overload_unused &) {
 template <typename T> enable_if_t<!cast_is_temporary_value_reference<T>::value, T> cast_ref(object &&, overload_unused &) {
     pybind11_fail("Internal error: cast_ref fallback invoked"); }
     pybind11_fail("Internal error: cast_ref fallback invoked"); }

include/pybind11/stl.h

@@ -53,7 +53,7 @@ template <typename Type, typename Key> struct set_caster {
         for (auto entry : s) {
         for (auto entry : s) {
             if (!conv.load(entry, convert))
             if (!conv.load(entry, convert))
                 return false;
                 return false;
-            value.insert(conv.operator typename key_conv::template cast_op_type<Key>());
+            value.insert(cast_op<Key>(conv));
         }
         }
         return true;
         return true;
     }
     }
@@ -87,9 +87,7 @@ template <typename Type, typename Key, typename Value> struct map_caster {
             if (!kconv.load(it.first.ptr(), convert) ||
             if (!kconv.load(it.first.ptr(), convert) ||
                 !vconv.load(it.second.ptr(), convert))
                 !vconv.load(it.second.ptr(), convert))
                 return false;
                 return false;
-            value.emplace(
-                    kconv.operator typename   key_conv::template cast_op_type<Key>(),
-                    vconv.operator typename value_conv::template cast_op_type<Value>());
+            value.emplace(cast_op<Key>(kconv), cast_op<Value>(vconv));
         }
         }
         return true;
         return true;
     }
     }
@@ -123,7 +121,7 @@ template <typename Type, typename Value> struct list_caster {
         for (auto it : s) {
         for (auto it : s) {
             if (!conv.load(it, convert))
             if (!conv.load(it, convert))
                 return false;
                 return false;
-            value.push_back(conv.operator typename value_conv::template cast_op_type<Value>());
+            value.push_back(cast_op<Value>(conv));
         }
         }
         return true;
         return true;
     }
     }
@@ -169,7 +167,7 @@ template <typename Type, size_t Size> struct type_caster<std::array<Type, Size>>
         for (auto it : l) {
         for (auto it : l) {
             if (!conv.load(it, convert))
             if (!conv.load(it, convert))
                 return false;
                 return false;
-            value[ctr++] = conv.operator typename value_conv::template cast_op_type<Type>();
+            value[ctr++] = cast_op<Type>(conv);
         }
         }
         return true;
         return true;
     }
     }
@@ -221,7 +219,7 @@ template<typename T> struct optional_caster {
         if (!inner_caster.load(src, convert))
         if (!inner_caster.load(src, convert))
             return false;
             return false;
 
 
-        value.emplace(inner_caster.operator typename value_conv::template cast_op_type<typename T::value_type>());
+        value.emplace(cast_op<typename T::value_type>(inner_caster));
         return true;
         return true;
     }
     }