math/big: round x + (-x) to -0 for mode ToNegativeInf

[bmkessler]

The source of float.go contains the following comment about handling the sign bit for Add/Sub

// Handling of sign bit as defined by IEEE 754-2008, section 6.3:
//
// When neither the inputs nor result are NaN, the sign of a product or
// quotient is the exclusive OR of the operands’ signs; the sign of a sum,
// or of a difference x−y regarded as a sum x+(−y), differs from at most
// one of the addends’ signs; and the sign of the result of conversions,
// the quantize operation, the roundToIntegral operations, and the
// roundToIntegralExact (see 5.3.1) is the sign of the first or only operand.
// These rules shall apply even when operands or results are zero or infinite.
//
// When the sum of two operands with opposite signs (or the difference of
// two operands with like signs) is exactly zero, the sign of that sum (or
// difference) shall be +0 in all rounding-direction attributes except
// roundTowardNegative; under that attribute, the sign of an exact zero
// sum (or difference) shall be −0. However, x+x = x−(−x) retains the same
// sign as x even when x is zero.
//
// See also: https://play.golang.org/p/RtH3UCt5IH

And the documentation for func (z *Float) Add(x, y *Float) *Float notes that rounding mode ToNegativeInf does not correctly follow this spec.

// BUG(gri) When rounding ToNegativeInf, the sign of Float values rounded to 0 is incorrect.

@griesemer

[gopherbot]

Change https://golang.org/cl/117495 mentions this issue: math/big: round x + (-x) to -0 for mode ToNegativeInf