math/big: fix Add, Sub when receiver aliases 2nd operand

Fixes #20490

Change-Id: I9cfa604f9ff94df779cb9b4cbbd706258fc473ac
Reviewed-on: https://go-review.googlesource.com/44150
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/math/big/float.go

@@ -1439,8 +1439,16 @@ func (z *Float) Add(x, y *Float) *Float {
 
 	if x.form == finite && y.form == finite {
 		// x + y (common case)
+
+		// Below we set z.neg = x.neg, and when z aliases y this will
+		// change the y operand's sign. This is fine, because if an
+		// operand aliases the receiver it'll be overwritten, but we still
+		// want the original x.neg and y.neg values when we evaluate
+		// x.neg != y.neg, so we need to save y.neg before setting z.neg.
+		yneg := y.neg
+
 		z.neg = x.neg
-		if x.neg == y.neg {
+		if x.neg == yneg {
 			// x + y == x + y
 			// (-x) + (-y) == -(x + y)
 			z.uadd(x, y)
@@ -1502,8 +1510,9 @@ func (z *Float) Sub(x, y *Float) *Float {
 
 	if x.form == finite && y.form == finite {
 		// x - y (common case)
+		yneg := y.neg
 		z.neg = x.neg
-		if x.neg != y.neg {
+		if x.neg != yneg {
 			// x - (-y) == x + y
 			// (-x) - y == -(x + y)
 			z.uadd(x, y)

src/math/big/float_test.go

@@ -1325,6 +1325,34 @@ func TestFloatAdd64(t *testing.T) {
 	}
 }
 
+func TestIssue20490(t *testing.T) {
+	var tests = []struct {
+		a, b float64
+	}{
+		{4, 1},
+		{-4, 1},
+		{4, -1},
+		{-4, -1},
+	}
+
+	for _, test := range tests {
+		a, b := NewFloat(test.a), NewFloat(test.b)
+		diff := new(Float).Sub(a, b)
+		b.Sub(a, b)
+		if b.Cmp(diff) != 0 {
+			t.Errorf("got %g - %g = %g; want %g\n", a, NewFloat(test.b), b, diff)
+		}
+
+		b = NewFloat(test.b)
+		sum := new(Float).Add(a, b)
+		b.Add(a, b)
+		if b.Cmp(sum) != 0 {
+			t.Errorf("got %g + %g = %g; want %g\n", a, NewFloat(test.b), b, sum)
+		}
+
+	}
+}
+
 // TestFloatMul tests Float.Mul/Quo by comparing the result of a "manual"
 // multiplication/division of arguments represented by Bits values with the
 // respective Float multiplication/division for a variety of precisions