testing/quick: use Uint64 instead of Int63

Followup to CL 39152.

Change-Id: I9bfed0c6071ea3d3a43294a6c4a50edc131368cf
Reviewed-on: https://go-review.googlesource.com/39313
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/testing/quick/quick.go

@@ -46,11 +46,7 @@ func randFloat64(rand *rand.Rand) float64 {
 
 // 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
+	return int64(rand.Uint64())
 }
 
 // complexSize is the maximum length of arbitrary values that contain other

src/testing/quick/quick_test.go

@@ -307,3 +307,21 @@ func TestNonZeroSliceAndMap(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func TestInt64(t *testing.T) {
+	var lo, hi int64
+	f := func(x int64) bool {
+		if x < lo {
+			lo = x
+		}
+		if x > hi {
+			hi = x
+		}
+		return true
+	}
+	cfg := &Config{MaxCount: 100000}
+	Check(f, cfg)
+	if uint64(lo)>>62 == 0 || uint64(hi)>>62 == 0 {
+		t.Errorf("int64 returned range %#016x,%#016x; does not look like full range", lo, hi)
+	}
+}