GNAT 4.2.3 Ubuntu reports weird compile error wrt Unchecked_Access

[Jerry <lanceboyle@qwest.net>]

I have a curious report from two members of the development team for
PLplot, a popular plotting package for which I have written bindings
in Ada. The project is mature enough that the team decided to enable
Ada by default when building, so I am concerned about the following e-
mail.

========== begin e-mail ============

<First comment from team member>

This latest change to the Ada bindings causes a build error for me
with gnat 4.2.3 on Ubuntu.

plplot.adb:2751:37: non-local pointer cannot point to local object
The offending line is
    PL_Pen_Labels(Index) := Temp_C_String'Unchecked_Access;

<Response from another team member>

This must depend on the version of gnat since I don't see this error
for gnat 4.3.1-2 from Debian testing (and I presume Jerry didn't see
it for his gnat-4.3 version as well).

========= end e-mail ==============

I (Jerry) in fact did not see a problem on my OS X 4.3 compiler.

I could supply more details of the code and might even try to write a
small program but testing on someone else's machine would be a bit
tedious. BUT--this error makes no sense to me--I would expect to (and
_do_) see this error if the line in question is

    PL_Pen_Labels(Index) := Temp_C_String'Access;

The whole purpose of using Unchecked_Access is to make this error go
away. It this a known problem on 4.2.3 Unbutu? Is it a compiler error?
Why does it work as expected on my 4.3 OS X, on 4.3.1-2 Debian
testing, but not 4.2.3 Ubuntu?

Jerry