private static void ensureNoMoreObservableSource(Type returnType, Type type) {
        if (type instanceof ParameterizedType) {
            final Type[] args = ((ParameterizedType) type).getActualTypeArguments();
            for (Type arg : args) {
                final Class<?> clazz = typeToClass(arg);
                if (clazz != null && isSupportedClass(clazz)) {
                    throw new IllegalStateException(
                            "Disallowed type exists in the generic type arguments of the return type '" +
                            returnType + "': " + clazz.getName());
                }
                ensureNoMoreObservableSource(returnType, arg);
            }
        }
    }
Provides an {@link ObservableResponseConverterFunction} to annotated services.
rxjava2/src/main/java/com/linecorp/armeria/internal/server/rxjava2/ObservableResponseConverterFunctionProvider.java