big: handle aliasing correctly for Rat.SetFrac.

Fixes #2379.

R=r, rsc
CC=golang-dev
https://golang.org/cl/5305043

src/pkg/big/rat.go

@@ -27,9 +27,13 @@ func NewRat(a, b int64) *Rat {
 
 // SetFrac sets z to a/b and returns z.
 func (z *Rat) SetFrac(a, b *Int) *Rat {
-	z.a.Set(a)
 	z.a.neg = a.neg != b.neg
-	z.b = z.b.set(b.abs)
+	babs := b.abs
+	if &z.a == b || alias(z.a.abs, babs) {
+		babs = nat(nil).set(babs) // make a copy
+	}
+	z.a.abs = z.a.abs.set(a.abs)
+	z.b = z.b.set(babs)
 	return z.norm()
 }
 

src/pkg/big/rat_test.go

@@ -330,3 +330,43 @@ func TestRatGobEncoding(t *testing.T) {
 		}
 	}
 }
+
+func TestIssue2379(t *testing.T) {
+	// 1) no aliasing
+	q := NewRat(3, 2)
+	x := new(Rat)
+	x.SetFrac(NewInt(3), NewInt(2))
+	if x.Cmp(q) != 0 {
+		t.Errorf("1) got %s want %s", x, q)
+	}
+
+	// 2) aliasing of numerator
+	x = NewRat(2, 3)
+	x.SetFrac(NewInt(3), x.Num())
+	if x.Cmp(q) != 0 {
+		t.Errorf("2) got %s want %s", x, q)
+	}
+
+	// 3) aliasing of denominator
+	x = NewRat(2, 3)
+	x.SetFrac(x.Denom(), NewInt(2))
+	if x.Cmp(q) != 0 {
+		t.Errorf("3) got %s want %s", x, q)
+	}
+
+	// 4) aliasing of numerator and denominator
+	x = NewRat(2, 3)
+	x.SetFrac(x.Denom(), x.Num())
+	if x.Cmp(q) != 0 {
+		t.Errorf("4) got %s want %s", x, q)
+	}
+
+	// 5) numerator and denominator are the same
+	q = NewRat(1, 1)
+	x = new(Rat)
+	n := NewInt(7)
+	x.SetFrac(n, n)
+	if x.Cmp(q) != 0 {
+		t.Errorf("5) got %s want %s", x, q)
+	}
+}