2 files changed, 35 insertions, 6 deletions

diff --git a/src/math/big/rat.go b/src/math/big/rat.go
index ee8af1456d..d35cd4cbd1 100644
--- a/src/math/big/rat.go
+++ b/src/math/big/rat.go
@@ -411,12 +411,19 @@ func (x *Rat) Num() *Int {
 }
 
 // Denom returns the denominator of x; it is always > 0.
-// The result is a reference to x's denominator; it
+// The result is a reference to x's denominator, unless
+// x is an uninitialized (zero value) Rat, in which case
+// the result is a new Int of value 1. (To initialize x,
+// any operation that sets x will do, including x.Set(x).)
+// If the result is a reference to x's denominator it
 // may change if a new value is assigned to x, and vice versa.
 func (x *Rat) Denom() *Int {
 	x.b.neg = false // the result is always >= 0
 	if len(x.b.abs) == 0 {
-		x.b.abs = x.b.abs.set(natOne) // materialize denominator (see issue #33792)
+		// Note: If this proves problematic, we could
+		//       panic instead and require the Rat to
+		//       be explicitly initialized.
+		return &Int{abs: nat{1}}
 	}
 	return &x.b
 }
diff --git a/src/math/big/rat_test.go b/src/math/big/rat_test.go
index 35bc85c8cd..02569c1b16 100644
--- a/src/math/big/rat_test.go
+++ b/src/math/big/rat_test.go
@@ -329,18 +329,40 @@ func TestIssue3521(t *testing.T) {
 		t.Errorf("0) got %s want %s", zero.Denom(), one)
 	}
 
-	// 1a) a zero value remains zero independent of denominator
+	// 1a) the denominator of an (uninitialized) zero value is not shared with the value
+	s := &zero.b
+	d := zero.Denom()
+	if d == s {
+		t.Errorf("1a) got %s (%p) == %s (%p) want different *Int values", d, d, s, s)
+	}
+
+	// 1b) the denominator of an (uninitialized) value is a new 1 each time
+	d1 := zero.Denom()
+	d2 := zero.Denom()
+	if d1 == d2 {
+		t.Errorf("1b) got %s (%p) == %s (%p) want different *Int values", d1, d1, d2, d2)
+	}
+
+	// 1c) the denominator of an initialized zero value is shared with the value
 	x := new(Rat)
+	x.Set(x) // initialize x (any operation that sets x explicitly will do)
+	s = &x.b
+	d = x.Denom()
+	if d != s {
+		t.Errorf("1c) got %s (%p) != %s (%p) want identical *Int values", d, d, s, s)
+	}
+
+	// 1d) a zero value remains zero independent of denominator
 	x.Denom().Set(new(Int).Neg(b))
 	if x.Cmp(zero) != 0 {
-		t.Errorf("1a) got %s want %s", x, zero)
+		t.Errorf("1d) got %s want %s", x, zero)
 	}
 
-	// 1b) a zero value may have a denominator != 0 and != 1
+	// 1e) a zero value may have a denominator != 0 and != 1
 	x.Num().Set(a)
 	qab := new(Rat).SetFrac(a, b)
 	if x.Cmp(qab) != 0 {
-		t.Errorf("1b) got %s want %s", x, qab)
+		t.Errorf("1e) got %s want %s", x, qab)
 	}
 
 	// 2a) an integral value becomes a fraction depending on denominator