big: bug fix for Quo aliasing problem

Fixes #820.

R=rsc
CC=golang-dev
https://golang.org/cl/1453041

src/pkg/big/rat.go

@@ -160,9 +160,11 @@ func (z *Rat) Quo(x, y *Rat) *Rat {
 	if len(y.a.abs) == 0 {
 		panic("division by zero")
 	}
-	z.a.abs = z.a.abs.mul(x.a.abs, y.b)
-	z.b = z.b.mul(x.b, y.a.abs)
-	z.a.neg = x.a.neg != y.a.neg
+	a := mulNat(&x.a, y.b)
+	b := mulNat(&y.a, x.b)
+	z.a.abs = a.abs
+	z.b = b.abs
+	z.a.neg = a.neg != b.neg
 	return z.norm()
 }
 

src/pkg/big/rat_test.go

@@ -175,3 +175,29 @@ func TestRatBin(t *testing.T) {
 		}
 	}
 }
+
+
+func TestIssue820(t *testing.T) {
+	x := NewRat(3, 1)
+	y := NewRat(2, 1)
+	z := y.Quo(x, y)
+	q := NewRat(3, 2)
+	if z.Cmp(q) != 0 {
+		t.Errorf("got %s want %s", z, q)
+	}
+
+	y = NewRat(3, 1)
+	x = NewRat(2, 1)
+	z = y.Quo(x, y)
+	q = NewRat(2, 3)
+	if z.Cmp(q) != 0 {
+		t.Errorf("got %s want %s", z, q)
+	}
+
+	x = NewRat(3, 1)
+	z = x.Quo(x, x)
+	q = NewRat(3, 3)
+	if z.Cmp(q) != 0 {
+		t.Errorf("got %s want %s", z, q)
+	}
+}