math/big: don't return io.EOF on successful call of ParseFloat

Fixes $9938.

Change-Id: Ie8680a875225748abd660fb26b4c25546e7b92d3
Reviewed-on: https://go-review.googlesource.com/5620Reviewed-by: Alan Donovan <adonovan@google.com>

src/math/big/floatconv.go

@@ -126,11 +126,9 @@ func (z *Float) Scan(r io.ByteScanner, base int) (f *Float, b int, err error) {
 }
 
 // Parse is like z.Scan(r, base), but instead of reading from an
-// io.ByteScanner, it parses the string s. An error is returned if the
-// string contains invalid or trailing characters not belonging to the
-// number.
-//
-// TODO(gri) define possible errors more precisely
+// io.ByteScanner, it parses the string s. An error is returned if
+// the string contains invalid or trailing bytes not belonging to
+// the number.
 func (z *Float) Parse(s string, base int) (f *Float, b int, err error) {
 	r := strings.NewReader(s)
 
@@ -139,11 +137,10 @@ func (z *Float) Parse(s string, base int) (f *Float, b int, err error) {
 	}
 
 	// entire string must have been consumed
-	var ch byte
-	if ch, err = r.ReadByte(); err != io.EOF {
-		if err == nil {
+	if ch, err2 := r.ReadByte(); err2 == nil {
 		err = fmt.Errorf("expected end of string, found %q", ch)
-		}
+	} else if err2 != io.EOF {
+		err = err2
 	}
 
 	return

src/math/big/floatconv_test.go

@@ -5,7 +5,6 @@
 package big
 
 import (
-	"io"
 	"math"
 	"strconv"
 	"testing"
@@ -59,7 +58,7 @@ func TestFloatSetFloat64String(t *testing.T) {
 		{"+10000000000000000000000000000000000000000e-0", 1e40},
 	} {
 		var x Float
-		x.prec = 53 // TODO(gri) find better solution
+		x.SetPrec(53)
 		_, ok := x.SetString(test.s)
 		if !ok {
 			t.Errorf("%s: parse error", test.s)
@@ -313,8 +312,7 @@ func TestFloatFormat(t *testing.T) {
 		{"3.14", 'x', 0, "%x"},
 	} {
 		f, _, err := ParseFloat(test.x, 0, 1000, ToNearestEven)
-		// TODO(gri) should we return io.EOF at the end?
-		if err != nil && err != io.EOF {
+		if err != nil {
 			t.Errorf("%v: %s", test, err)
 			continue
 		}