Add caster_odr_decltype.

Models pybind11_select_caster (pybind/pybind11#3931) instead of the older type_caster specialization mechanics.

Author: rwgk

caster_odr_decltype/car.cc

@@ -0,0 +1,15 @@
+#include "engine.h"
+
+namespace vehicles {
+
+struct vehicle;
+
+struct car_specs {
+  int power() const { return 100; }
+};
+
+car_specs engine_select_specs(vehicle *);
+
+}  // namespace vehicles
+
+int car_power() { return engine::power<vehicles::vehicle>(); }

caster_odr_decltype/compile_and_run.bat

@@ -0,0 +1,8 @@
+cl car.cc truck.cc vehicle_specs.cc /Fe"show.exe"
+.\show
+cl truck.cc car.cc vehicle_specs.cc /Fe"show.exe"
+.\show
+cl /DUSE_UNIQUE_TO_TRANSLATION_UNIT car.cc truck.cc vehicle_specs.cc /Fe"show.exe"
+.\show
+cl /DUSE_UNIQUE_TO_TRANSLATION_UNIT truck.cc car.cc vehicle_specs.cc /Fe"show.exe"
+.\show

caster_odr_decltype/compile_and_run.sh

@@ -0,0 +1,7 @@
+set -e
+set -x
+clang++ -std=c++11 car.cc truck.cc vehicle_specs.cc && ./a.out
+clang++ -std=c++11 truck.cc car.cc vehicle_specs.cc && ./a.out
+clang++ -std=c++11 -DUSE_UNIQUE_TO_TRANSLATION_UNIT car.cc truck.cc vehicle_specs.cc && ./a.out
+clang++ -std=c++11 -DUSE_UNIQUE_TO_TRANSLATION_UNIT truck.cc car.cc vehicle_specs.cc && ./a.out
+rm a.out

caster_odr_decltype/engine.h

@@ -0,0 +1,21 @@
+namespace engine {
+
+#ifdef USE_UNIQUE_TO_TRANSLATION_UNIT
+
+namespace {
+struct unique_to_translation_unit {};
+}  // namespace
+
+#define TTT template <typename T, typename U = unique_to_translation_unit>
+
+#else
+
+#define TTT template <typename T>
+
+#endif
+
+TTT using make_specs = decltype(engine_select_specs(static_cast<T *>(nullptr)));
+
+TTT int power() { return make_specs<T>().power(); }
+
+}  // namespace engine

caster_odr_decltype/truck.cc

@@ -0,0 +1,15 @@
+#include "engine.h"
+
+namespace vehicles {
+
+struct vehicle;
+
+struct truck_specs {
+  int power() const { return 300; }
+};
+
+truck_specs engine_select_specs(vehicle *);
+
+}  // namespace vehicles
+
+int truck_power() { return engine::power<vehicles::vehicle>(); }

caster_odr_decltype/vehicle_specs.cc

@@ -0,0 +1,10 @@
+#include <iostream>
+
+int car_power();
+int truck_power();
+
+int main() {
+  std::cout << "car   power:   " << car_power() << std::endl;
+  std::cout << "truck power:   " << truck_power() << std::endl;
+  return 0;
+}