testing/quick: generate all possible int64, uint64 values

When generating a random int8, uint8, int16, uint16, int32, uint32,
quick.Value chooses among all possible values.

But when generating a random int64 or uint64, it only chooses
values in the range [-2⁶², 2⁶²) (even for uint64).
It should, like for all the other integers, use the full range.

If it had, this would have caught #19807 earlier.
Instead it let us discover the presence of #19809.

While we are here, also make the default source of
randomness not completely deterministic.

Fixes #19808.

Change-Id: I070f852531c92b3670bd76523326c9132bfc9416
Reviewed-on: https://go-review.googlesource.com/39152
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/testing/quick/quick.go

@@ -14,6 +14,7 @@ import (
 	"math/rand"
 	"reflect"
 	"strings"
+	"time"
 )
 
 var defaultMaxCount *int = flag.Int("quickchecks", 100, "The default number of iterations for each check")
@@ -43,8 +44,14 @@ func randFloat64(rand *rand.Rand) float64 {
 	return f
 }
 
-// randInt64 returns a random integer taking half the range of an int64.
-func randInt64(rand *rand.Rand) int64 { return rand.Int63() - 1<<62 }
+// randInt64 returns a random int64.
+func randInt64(rand *rand.Rand) int64 {
+	x := rand.Int63() - 1<<62
+	// x in [-2⁶²,2⁶²), so top two bits are 00 or 11, never 10 or 01.
+	// Mix in some bits from the middle.
+	x ^= x<<29 ^ x<<43
+	return x
+}
 
 // complexSize is the maximum length of arbitrary values that contain other
 // values.
@@ -193,7 +200,7 @@ var defaultConfig Config
 // getRand returns the *rand.Rand to use for a given Config.
 func (c *Config) getRand() *rand.Rand {
 	if c.Rand == nil {
-		return rand.New(rand.NewSource(0))
+		return rand.New(rand.NewSource(time.Now().UnixNano()))
 	}
 	return c.Rand
 }