Created attachment 27446 [details]
source file exhibiting the bug when compiled

Compiling attached main.cpp gives the following error:

jeffrey@ubuntu:~/scratch$ g++ -o scratch main.cpp
main.cpp: In function ‘int main(int, char**)’:
main.cpp:21:6: error: ‘operator Y<X>&’ is not a member of ‘X’

However, no error occurs with any of the following changes:
(a) Removing the __attribute__((__may_alias__)) (commenting in line 3 and commenting out line 4).
(b) Removing the implementation (line 8) or using a throwing implementation (line 9) rather than the present implementation (line 10).
(c) Replacing the conversion operator with a regular member function (commenting out lines 10,21 and commenting in lines 11,22 or 12,22.

Further investigation reveals a possibly related error. One can define the function out-of-line (commenting in lines 8,16 and commenting out line 10) with no error, as long as line 15 remains commented out; additionally commenting in line 15 gives the following error:

jeffrey@ubuntu:~/scratch$ g++ -o scratch main.cpp
main.cpp:16:20: error: no ‘X::operator Y<X>&()’ member function declared in class ‘X’

This error still appears if one uses a throwing implementation (like in (b)) or replaces the conversion operator with a regular member function (like in (c)), but no error occurs with the removal of the __attribute__((__may_alias__)) (as in (a)).