PdmObjectCollection: Move PdmObject base check to constructor

Replace the class-level static_assert with a DerivedFromPdmObject concept
checked inside the constructor body. The constructor is lazily instantiated,
so T can be forward-declared at the point PdmObjectCollection<T> is used
as a base class.

Fwk/AppFwk/cafProjectDataModel/cafPdmObjectCollection.h

@@ -21,11 +21,24 @@
 #include "cafPdmChildArrayField.h"
 #include "cafPdmObject.h"
 
+#include <concepts>
 #include <vector>
 
 namespace caf
 {
 
+//==================================================================================================
+///
+/// Constrains a template parameter to types deriving from caf::PdmObject.
+///
+/// Checked via `static_assert` inside a lazily-instantiated member function (the constructor)
+/// rather than as a class-template constraint, so that T may be forward-declared at the point
+/// the class template is instantiated as a base class.
+///
+//==================================================================================================
+template <typename T>
+concept DerivedFromPdmObject = std::derived_from<T, PdmObject>;
+
 //==================================================================================================
 ///
 /// Generic templated collection base class for managing collections of PDM objects
@@ -42,8 +55,6 @@ namespace caf
 template <typename T>
 class PdmObjectCollection : public PdmObject
 {
-    static_assert( std::is_base_of<caf::PdmObject, T>::value, "T must inherit from caf::PdmObject" );
-
 public:
     // Collection access
     std::vector<T*>               items() const;
@@ -89,4 +100,4 @@ private:
 } // namespace caf
 
 // Include template implementation
-#include "cafPdmObjectCollection.inl"
+#include "cafPdmObjectCollection.inl"

Fwk/AppFwk/cafProjectDataModel/cafPdmObjectCollection.inl

@@ -29,6 +29,8 @@ namespace caf
 template <typename T>
 PdmObjectCollection<T>::PdmObjectCollection()
 {
+    static_assert( DerivedFromPdmObject<T>, "T must inherit from caf::PdmObject" );
+
     // m_items field must be initialized by derived class using CAF_PDM_InitFieldNoDefault
 }