Add custom std::function<> caster.

tests/python/test_functional.cpp

@@ -0,0 +1,38 @@
+#include "pybind11_utils/functional.h"
+
+#include <pybind11/pybind11.h>
+
+PYBIND11_MODULE(functional, m)
+{
+    m.def("fn_0_arg", [](std::function<int()> const& fn) {
+        return fn();
+    });
+
+    m.def("fn_1_arg", [](std::function<int(int)> const& fn, int a) {
+        return fn(a);
+    });
+
+    m.def("fn_2_arg", [](std::function<int(int, int)> const& fn, int a, int b) {
+        return fn(a, b);
+    });
+
+    m.def("fn_0_or_1_arg", [](std::function<int()> const& fn) {
+        return fn();
+    });
+
+    m.def("fn_0_or_1_arg", [](std::function<int(int)> const& fn) {
+        return fn(1);
+    });
+
+    m.def("fn_1_or_2_or_3_arg", [](std::function<int(int)> const& fn) {
+        return fn(1);
+    });
+
+    m.def("fn_1_or_2_or_3_arg", [](std::function<int(int, int)> const& fn) {
+        return fn(1, 2);
+    });
+
+    m.def("fn_1_or_2_or_3_arg", [](std::function<int(int, int, int)> const& fn) {
+        return fn(1, 2, 3);
+    });
+}