Add power_less(), more_power_less()

```
$ ./compile_and_run.sh
++ clang++ -std=c++11 unknown.cc car.cc truck.cc vehicle_specs.cc
++ ./a.out
unknown power:   0
car   power: 0  100
truck power: 0  300
car   power: 1  100
truck power: 1  100
car   power: 2  100
truck power: 2  100
++ clang++ -std=c++11 unknown.cc truck.cc car.cc vehicle_specs.cc
++ ./a.out
unknown power:   0
car   power: 0  100
truck power: 0  300
car   power: 1  300
truck power: 1  300
car   power: 2  300
truck power: 2  300
++ rm a.out
```

Author: rwgk

caster_odr_specs_scope/car.cc

@@ -23,6 +23,13 @@ struct specs_scope<vehicles::vehicle, unique_to_translation_unit> {
 
 }  // namespace engine
 
-int car_power() {
-  return engine::power<vehicles::vehicle, engine::unique_to_translation_unit>();
+int car_power(int i) {
+  if (i == 0) {
+    return engine::power<vehicles::vehicle,
+                         engine::unique_to_translation_unit>();
+  }
+  if (i == 1) {
+    return engine::power_less<vehicles::vehicle>();
+  }
+  return engine::more_power_less<vehicles::vehicle>();
 }

caster_odr_specs_scope/engine.h

@@ -24,4 +24,16 @@ int power() {
   return make_specs<T, typename specs_scope<T, U>::select>().power();
 }
 
+template <typename T>
+int power_less() { // Violates the ODR.
+  return make_specs<
+             T, typename specs_scope<T, unique_to_translation_unit>::select>()
+      .power();
+}
+
+template <typename T>
+int more_power_less() { // This, too, transitively.
+  return power_less<T>();
+}
+
 }  // namespace engine

caster_odr_specs_scope/truck.cc

@@ -23,6 +23,13 @@ struct specs_scope<vehicles::vehicle, unique_to_translation_unit> {
 
 }  // namespace engine
 
-int truck_power() {
-  return engine::power<vehicles::vehicle, engine::unique_to_translation_unit>();
+int truck_power(int i) {
+  if (i == 0) {
+    return engine::power<vehicles::vehicle,
+                         engine::unique_to_translation_unit>();
+  }
+  if (i == 1) {
+    return engine::power_less<vehicles::vehicle>();
+  }
+  return engine::more_power_less<vehicles::vehicle>();
 }

caster_odr_specs_scope/vehicle_specs.cc

@@ -1,12 +1,14 @@
 #include <iostream>
 
 int unknown_power();
-int car_power();
-int truck_power();
+int car_power(int i);
+int truck_power(int i);
 
 int main() {
   std::cout << "unknown power:   " << unknown_power() << std::endl;
-  std::cout << "car     power:   " << car_power() << std::endl;
-  std::cout << "truck   power:   " << truck_power() << std::endl;
+  for (int i = 0; i < 3; i++) {
+    std::cout << "car   power: " << i << "  " << car_power(i) << std::endl;
+    std::cout << "truck power: " << i << "  " << truck_power(i) << std::endl;
+  }
   return 0;
 }