Prevent creation of circular dependencies in constructors

TUTORIAL.md

@@ -137,7 +137,7 @@ Constructor injection has the advantage of not having to import Poodinis through
 
 Circular dependencies
 ---------------------
-Poodinis can autowire circular dependencies when they are registered with `singleInstance` or `existingInstance` registration scopes. Circular dependencies in registrations with `newInstance` scopes will not be autowired, as this would cause an endless loop.
+Poodinis can autowire circular dependencies when they are registered with `singleInstance` or `existingInstance` registration scopes. Circular dependencies in registrations with `newInstance` scopes will not be autowired, as this would cause an endless loop. Circular dependencies are only supported when autowiring members through the `@Autowire` UDA; circular dependencies in constructors are not supported and will result in an `InstanceCreationException`.
 
 Registering and resolving using qualifiers
 ------------------------------------------

source/poodinis/factory.d

@@ -89,6 +89,7 @@ class InstanceFactory {
 
 class ConstructorInjectingInstanceFactory(InstanceType) : InstanceFactory {
 	private shared DependencyContainer container;
+	private bool isBeingInjected = false;
 
 	this(shared DependencyContainer container) {
 		this.container = container;
@@ -120,15 +121,18 @@ class ConstructorInjectingInstanceFactory(InstanceType) : InstanceFactory {
 
 	protected override Object createInstance() {
 		enforce!InstanceCreationException(container, "A dependency container is not defined. Cannot perform constructor injection without one.");
+		enforce!InstanceCreationException(!isBeingInjected, format("%s is already being created and injected; possible circular dependencies in constructors?", InstanceType.stringof));
 
 		Object instance = null;
 		static if (__traits(compiles, __traits(getOverloads, InstanceType, `__ctor`))) {
 			foreach(ctor ; __traits(getOverloads, InstanceType, `__ctor`)) {
 				static if (parametersAreValid!(Parameters!ctor)) {
+					isBeingInjected = true;
 					mixin(`
 						import ` ~ moduleName!InstanceType ~ `;
 						instance = new ` ~ fullyQualifiedName!InstanceType ~ `(` ~ createArgumentList!(Parameters!ctor) ~ `);
 					`);
+					isBeingInjected = false;
 					break;
 				}
 			}

test/poodinis/containertest.d

@@ -199,6 +199,14 @@ version(unittest) {
 		}
 	}
 
+	class Pot {
+		this(Kettle kettle) {}
+	}
+
+	class Kettle {
+		this(Pot pot) {}
+	}
+
 	// Test register concrete type
 	unittest {
 		auto container = new shared DependencyContainer();
@@ -695,4 +703,13 @@ version(unittest) {
 		assert(instance !is null);
 		assert(instance.color is container.resolve!Blue);
 	}
+
+	// Test prevention of circular dependencies during constructor injection
+	unittest {
+		auto container = new shared DependencyContainer();
+		container.register!Pot;
+		container.register!Kettle;
+
+		assertThrown!InstanceCreationException(container.resolve!Pot);
+	}
 }