Trying to override construct-only properties always fails when the overriding implementation is in a different file to the original definition. This is the case both for interfaces and for base classes defining a virtual/abstract construct-only property. An example using an interface:

File 1:

public interface Test.Iface : Object
{
    public abstract int prop { get; construct; }
}

File 2:

public class Test.Impl : Object, Iface
{
    public int prop { get; construct; }
    
    public Impl()
    {
        Object(prop: 3);
    }
}

void main()
{
    new Test.Impl();
}

(Replace the interface with a base class with a virtual property and the result is the same.)

Vala compiles the code to C without warning, but the linker fails with the error "undefined reference to `test_iface_set_prop'".

Looking at the generated C code, there are a couple of funny things going on. Firstly, the implementing class registers its property with g_object_class_install_property(), whereas idiomatic GObject would be to use g_object_class_override_property().

Secondly, the set_property() function in the implementing class calls test_iface_set_prop() (a static function in the interface C file, causing the error). I can't see any reason why it should go through this virtual function rather than calling its own test_impl_real_set_prop() version.