a value composed of to with from's sign bit.
assert(copysign(1.0, 1.0) == 1.0); assert(copysign(1.0, -0.0) == -1.0); assert(copysign(1UL, -1.0) == -1.0); assert(copysign(-1.0, -1.0) == -1.0); assert(copysign(real.infinity, -1.0) == -real.infinity); assert(copysign(real.nan, 1.0) is real.nan); assert(copysign(-real.nan, 1.0) is real.nan); assert(copysign(real.nan, -1.0) is -real.nan);