The copy-to-selected operator for RNA buttons uses paths for copying
object pointer properties. Copying other ID data blocks is deliberately
disabled:
https://developer.blender.org/diffusion/B/browse/master/source/blender/editors/interface/interface_ops.c$274

However, the RNA_path_resolve_full function is not properly working for
retrieving pointer properties: it always will dereference pointer
properties in anticipation of further path elements.

In fact the return value of RNA_path_resolve_full has a conflicting
double meaning. It returns false when

• the RNA path is invalid
• any of the pointer properties is NULL

This means that it is not capable of returning pointer properties at all.
To make this possible, there is now an internal function for path
parsing, which returns false only if the the path is invalid.
On top of this there are 4 wrapper functions for retrieving either
actual property values (RNA_path_resolve, RNA_path_resolve_full) and for
retrieving pointer+property pairs (RNA_path_resolve_property,
RNA_path_resolve_property_full). The latter 2 variants will not
dereference pointer properties at the end of the path, so callers can
actually get the property itself. The ***_full variants include an
array index return value.