reflect: stop thinking that MaxFloat32 overflows float32.

Fixes #4282.

R=golang-dev, minux.ma, rsc
CC=golang-dev
https://golang.org/cl/6759052

src/pkg/reflect/all_test.go

@@ -2664,6 +2664,45 @@ func TestConvert(t *testing.T) {
 	}
 }
 
+func TestOverflow(t *testing.T) {
+	if ovf := V(float64(0)).OverflowFloat(1e300); ovf {
+		t.Errorf("%v wrongly overflows float64", 1e300)
+	}
+
+	maxFloat32 := float64((1<<24 - 1) << (127 - 23))
+	if ovf := V(float32(0)).OverflowFloat(maxFloat32); ovf {
+		t.Errorf("%v wrongly overflows float32", maxFloat32)
+	}
+	ovfFloat32 := float64((1<<24-1)<<(127-23) + 1<<(127-52))
+	if ovf := V(float32(0)).OverflowFloat(ovfFloat32); !ovf {
+		t.Errorf("%v should overflow float32", ovfFloat32)
+	}
+	if ovf := V(float32(0)).OverflowFloat(-ovfFloat32); !ovf {
+		t.Errorf("%v should overflow float32", -ovfFloat32)
+	}
+
+	maxInt32 := int64(0x7fffffff)
+	if ovf := V(int32(0)).OverflowInt(maxInt32); ovf {
+		t.Errorf("%v wrongly overflows int32", maxInt32)
+	}
+	if ovf := V(int32(0)).OverflowInt(-1 << 31); ovf {
+		t.Errorf("%v wrongly overflows int32", -int64(1)<<31)
+	}
+	ovfInt32 := int64(1 << 31)
+	if ovf := V(int32(0)).OverflowInt(ovfInt32); !ovf {
+		t.Errorf("%v should overflow int32", ovfInt32)
+	}
+
+	maxUint32 := uint64(0xffffffff)
+	if ovf := V(uint32(0)).OverflowUint(maxUint32); ovf {
+		t.Errorf("%v wrongly overflows uint32", maxUint32)
+	}
+	ovfUint32 := uint64(1 << 32)
+	if ovf := V(uint32(0)).OverflowUint(ovfUint32); !ovf {
+		t.Errorf("%v should overflow uint32", ovfUint32)
+	}
+}
+
 type B1 struct {
 	X int
 	Y int

src/pkg/reflect/value.go

@@ -1179,7 +1179,7 @@ func overflowFloat32(x float64) bool {
 	if x < 0 {
 		x = -x
 	}
-	return math.MaxFloat32 <= x && x <= math.MaxFloat64
+	return math.MaxFloat32 < x && x <= math.MaxFloat64
 }
 
 // OverflowInt returns true if the int64 x cannot be represented by v's type.