[propertysystem] Add Properties::hasDefinedType and GetPropOr/GetPropTypeOr

The former checks whether a property is specialized for the given typetag,
The latter are convenience alias to get the defined type if it's
specialized for the TypeTag and some other given type if not.

dumux/common/properties/propertysystem.hh

@@ -36,6 +36,10 @@ struct UndefinedProperty {};
 
 } // end namespace Dumux::Properties
 
+
+// hide from doxygen
+#ifndef DOXYGEN
+
 //! implementation details for template meta programming
 namespace Dumux::Properties::Detail {
 
@@ -124,22 +128,48 @@ struct GetDefined<TypeTag, Property, std::tuple<FirstTypeTag, Args...>>
                                      typename GetNextTypeTag<TypeTag, Property, std::tuple<FirstTypeTag, Args...>, void>::type>;
 };
 
-//! helper struct to extract get the Property specilization given a TypeTag, asserts that the property is defined
+//! helper struct to extract get the Property specialization given a TypeTag, asserts that the property is defined
 template<class TypeTag, template<class,class> class Property>
 struct GetPropImpl
 {
     using type = typename Detail::GetDefined<TypeTag, Property, std::tuple<TypeTag>>::type;
-    static_assert(!std::is_same<type, UndefinedProperty>::value, "Property is undefined!");
+    static_assert(!std::is_same_v<type, UndefinedProperty>, "Property is undefined!");
+};
+
+template<class TypeTag, template<class,class> class Property, class T>
+struct GetPropOrImpl
+{
+    using PT = typename Detail::GetDefined<TypeTag, Property, std::tuple<TypeTag>>::type;
+    struct OT { using type = T; }; // fake property wrapper
+    using type = std::conditional_t<std::is_same_v<PT, UndefinedProperty>, OT, PT>;
 };
 
 } // end namespace Dumux::Properties::Detail
 
+#endif // DOXYGEN
+
+namespace Dumux::Properties {
+
+//! whether the property is defined/specialized for TypeTag
+template<class TypeTag, template<class,class> class Property>
+inline constexpr bool hasDefinedType()
+{
+    using type = typename Detail::GetDefined<TypeTag, Property, std::tuple<TypeTag>>::type;
+    return !std::is_same_v<type, UndefinedProperty>;
+}
+
+} // end namespace Dumux::Properties
+
 namespace Dumux {
 
 //! get the type of a property
 template<class TypeTag, template<class,class> class Property>
 using GetProp = typename Properties::Detail::GetPropImpl<TypeTag, Property>::type;
 
+//! get the type of a property or the type T if the property is undefined
+template<class TypeTag, template<class,class> class Property, class T>
+using GetPropOr = typename Properties::Detail::GetPropOrImpl<TypeTag, Property, T>::type;
+
 // See the comment above.
 #ifdef __clang__
 #pragma clang diagnostic push
@@ -147,11 +177,15 @@ using GetProp = typename Properties::Detail::GetPropImpl<TypeTag, Property>::typ
 #endif
 //! get the type alias defined in the property
 template<class TypeTag, template<class,class> class Property>
-using GetPropType = typename Properties::Detail::GetPropImpl<TypeTag, Property>::type::type;
+using GetPropType = typename GetProp<TypeTag, Property>::type;
+
+//! get the type alias defined in the property or the type T if the property is undefined
+template<class TypeTag, template<class,class> class Property, class T>
+using GetPropTypeOr = typename GetPropOr<TypeTag, Property, T>::type;
 
 //! get the value data member of a property
 template<class TypeTag, template<class,class> class Property>
-constexpr auto getPropValue() { return Properties::Detail::GetPropImpl<TypeTag, Property>::type::value; }
+inline constexpr auto getPropValue() { return GetProp<TypeTag, Property>::value; }
 #ifdef __clang__
 #pragma clang diagnostic pop
 #endif