id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
12581	cpp_bin_float: Anal fixation. Part 4. Special cases of atan2()	Michael Shatz	John Maddock	"
In following special cases boost::atan2(y, x) return values are different from C RTL:

{{{
  y     x  C RTL  boost
 inf  inf  pi/4   nan
-inf  inf -pi/4   nan
 inf -inf  3*pi/4 nan
-inf -inf -3*pi/4 nan
-0   -val -pi     pi
 0    nan  nan    0
 inf  nan  nan    pi/2
-inf  nan  nan   -pi/2
}}}

The first four boost answer make sense. IMHO, they are better than C RTL answers.
The fifth boost answer is strange, but can be considered acceptable. 
The rest are plain bugs - nan should always propagate.
"	Feature Requests	closed	To Be Determined	multiprecision	Boost 1.62.0	Cosmetic	fixed	atan2