math/big: call norm when returning success from Rat SetString

After CL 24430, reflect.DeepEqual no longer returns true when comparing
a *Rat built with (*Rat).SetString("0") with one built with
(*Rat).SetInt64(0).
These should be equivalent, but because (*Rat).SetString does not call
norm() when returning the zero value, the result of reflect.DeepEqual
will be false.

One could suggest that developers should use (*Rat).Cmp instead
of relying on reflect.DeepEqual, but if a (*Rat) is part of a
larger struct that is being compared, this can be cumbersome.

This is fixed by calling z.norm() when returning zero in SetString.

Fixes #50944

Change-Id: Ib84ae975bf82fe02d1203aa9668a01960c0fd59d
Reviewed-on: https://go-review.googlesource.com/c/go/+/364434
Reviewed-by: Katie Hockman <[email protected]>
Trust: Katie Hockman <[email protected]>
Trust: Ian Lance Taylor <[email protected]>

src/math/big/ratconv.go

@@ -113,7 +113,7 @@ func (z *Rat) SetString(s string) (*Rat, bool) {
 
  // special-case 0 (see also issue #16176)
  if len(z.a.abs) == 0 {
- return z, true
+ return z.norm(), true
  }
  // len(z.a.abs) > 0
 

src/math/big/ratconv_test.go

@@ -9,6 +9,7 @@ import (
  "fmt"
  "io"
  "math"
+ "reflect"
  "strconv"
  "strings"
  "testing"
@@ -205,6 +206,14 @@ func TestRatSetString(t *testing.T) {
  }
 }
 
+func TestRatSetStringZero(t *testing.T) {
+ got, _ := new(Rat).SetString("0")
+ want := new(Rat).SetInt64(0)
+ if !reflect.DeepEqual(got, want) {
+ t.Errorf("got %#+v, want %#+v", got, want)
+ }
+}
+
 func TestRatScan(t *testing.T) {
  var buf bytes.Buffer
  for i, test := range setStringTests {