[Bug c++/58963] Does C++ need flag_complex_method = 2?

["congh at google dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58963

--- Comment #3 from Cong Hou <congh at google dot com> ---
Suppose there is a third-party complex library, which is written in the same
way as <complex>. Then GCC could not recognize that as complex type, and will
not use builtin calls to calculate multiplication and division. 

So why there should be a difference when I use the third-party complex lib and
the standard library lib. After all, <complex> is all written in source code.
<complex> is not the same as _Complex in C99.

If we can use _Complex in C++, it is fine. But C does not have <complex>: we
won't meet the situation that building the same file t.c using gcc and g++, and
g++ is faster. gcc cannot recognize <complex>.