id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
12039	cpp_bin_float convert_to<double>() rounding mistake	Michael Shatz <shatz@…>	John Maddock	"There are cases where cpp_bin_float convert_to<double>() does not produce the nearest 'double' result. It happens when the source argument differs form the middle point between two double-precision values in 65th bit or further.
I didn't look at boost source code, but pretty sure that the mistakes occurs due to double rounding. I.e. instead of direct rounding to 53-bit precision of 'double' the number is initially converted to 64-bit 'long double' and then converted from  'long double' to 'double'.

In particular, on Microsoft compilers 'long double' and 'double' refer to the same type and the mistake does not happen. 
But on the same machine/OS with gcc compiler the mistake does happen.

Below attached a simple test case that prints 'good' on platforms without bug and prints bad on platforms with bug.
"	Bugs	closed	To Be Determined	multiprecision	Boost 1.61.0	Problem	fixed