Fix for DR 925

[Paolo Carlini]

Hi,
> Hi, the attached patch fixes conversion from unique_ptr to shared_ptr,
> making it implicit, as per DR 925. Is it adequate to get applied to
> gcc-4.5? The patch is against gcc trunk as of 2010/03/18 (today).
>   
What about auto_ptr? Also, in the DR you provided a testcase, thus, why
not adding it to the testsuite? In general, patches come with a
ChangeLog entry.

Thanks,
Paolo.