math/big: better doc string for Float.Copy, add example test

Fixes #66358.

Change-Id: Ic9bde88eabfb2a446d32e1dc5ac404a51ef49f11
Reviewed-on: https://go-review.googlesource.com/c/go/+/590635
Auto-Submit: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>

src/math/big/float.go

@@ -672,9 +672,8 @@ func (z *Float) Set(x *Float) *Float {
 	return z
 }
 
-// Copy sets z to x, with the same precision, rounding mode, and
-// accuracy as x, and returns z. x is not changed even if z and
-// x are the same.
+// Copy sets z to x, with the same precision, rounding mode, and accuracy as x.
+// Copy returns z. If x and z are identical, Copy is a no-op.
 func (z *Float) Copy(x *Float) *Float {
 	if debugFloat {
 		x.validate()

src/math/big/floatexample_test.go

@@ -139,3 +139,24 @@ func ExampleRoundingMode() {
 	// -2.5             -2             -3      -2            -3             -3             -2
 	// -2.6             -3             -3      -2            -3             -3             -2
 }
+
+func ExampleFloat_Copy() {
+	var x, z big.Float
+
+	x.SetFloat64(1.23)
+	r := z.Copy(&x)
+	fmt.Printf("a) r = %g, z = %g, x = %g, r == z = %v\n", r, &z, &x, r == &z)
+
+	// changing z changes r since they are identical
+	z.SetInt64(42)
+	fmt.Printf("b) r = %g, z = %g, r == z = %v\n", r, &z, r == &z)
+
+	x.SetPrec(1)
+	z.Copy(&x)
+	fmt.Printf("c) z = %g, x = %g, z == x = %v\n", &z, &x, &z == &x)
+
+	// Output:
+	// a) r = 1.23, z = 1.23, x = 1.23, r == z = true
+	// b) r = 42, z = 42, r == z = true
+	// c) z = 1, x = 1, z == x = false
+}