reflect: make Value.IsZero identical to v == zero

The upcoming built-in zero value provides an idiomatic way
to test for zero by comparing to the zero literal: v == zero.

The reflect package is meant to provide a programmatic way to perform
operations that the Go language itself provides.
Thus, it seems prudent that reflect.ValueOf(&v).Elem().IsZero() is
identical to v == zero.

This change alters the behavior of Value.IsZero in two concrete ways:
* negative zero is identical to zero
* blank fields in a struct are ignored

Prior to this change, we were already in an inconsistent state
due to a regression introduced by CL 411478.
The new behavior was already the case for comparable composite types.
This change makes it consistent for all other types
(in particular incomparable composite types and standalone numbers).

Updates #61372
Fixes #61827

Change-Id: Id23fb97eb3b8921417cc75a1d3ead963e22dc3d9
Reviewed-on: https://go-review.googlesource.com/c/go/+/517777
Reviewed-by: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

doc/go1.22.html

@@ -100,6 +100,20 @@ Do not send CLs removing the interior tags from such phrases.
   </dd>
 </dl>
 
+<dl id="reflect"><dt><a href="/pkg/reflect/">reflect</a></dt>
+  <dd>
+    <p><!-- https://go.dev/issue/61827, CL 517777 -->
+      The <a href="/pkg/reflect/#Value.IsZero"><code>Value.IsZero</code></a>
+      method will now return true for a floating-point or complex
+      negative zero, and will return true for a struct value if a
+      blank field (a field named <code>_</code>) somehow has a
+      non-zero value.
+      These changes make <code>IsZero</code> consistent with comparing
+      a value to zero using the languague <code>==</code> operator.
+    </p>
+  </dd>
+</dl>
+
 <h2 id="ports">Ports</h2>
 
 <p>

src/reflect/all_test.go

@@ -1396,6 +1396,11 @@ func TestIsNil(t *testing.T) {
 	NotNil(fi, t)
 }
 
+func setField[S, V any](in S, offset uintptr, value V) (out S) {
+	*(*V)(unsafe.Add(unsafe.Pointer(&in), offset)) = value
+	return in
+}
+
 func TestIsZero(t *testing.T) {
 	for i, tt := range []struct {
 		x    any
@@ -1429,14 +1434,14 @@ func TestIsZero(t *testing.T) {
 		{float32(1.2), false},
 		{float64(0), true},
 		{float64(1.2), false},
-		{math.Copysign(0, -1), false},
+		{math.Copysign(0, -1), true},
 		{complex64(0), true},
 		{complex64(1.2), false},
 		{complex128(0), true},
 		{complex128(1.2), false},
-		{complex(math.Copysign(0, -1), 0), false},
-		{complex(0, math.Copysign(0, -1)), false},
-		{complex(math.Copysign(0, -1), math.Copysign(0, -1)), false},
+		{complex(math.Copysign(0, -1), 0), true},
+		{complex(0, math.Copysign(0, -1)), true},
+		{complex(math.Copysign(0, -1), math.Copysign(0, -1)), true},
 		{uintptr(0), true},
 		{uintptr(128), false},
 		// Array
@@ -1485,6 +1490,14 @@ func TestIsZero(t *testing.T) {
 		{struct{ s []int }{[]int{1}}, false},  // incomparable struct
 		{struct{ Value }{}, true},
 		{struct{ Value }{ValueOf(0)}, false},
+		{struct{ _, a, _ uintptr }{}, true}, // comparable struct with blank fields
+		{setField(struct{ _, a, _ uintptr }{}, 0*unsafe.Sizeof(uintptr(0)), 1), true},
+		{setField(struct{ _, a, _ uintptr }{}, 1*unsafe.Sizeof(uintptr(0)), 1), false},
+		{setField(struct{ _, a, _ uintptr }{}, 2*unsafe.Sizeof(uintptr(0)), 1), true},
+		{struct{ _, a, _ func() }{}, true}, // incomparable struct with blank fields
+		{setField(struct{ _, a, _ func() }{}, 0*unsafe.Sizeof((func())(nil)), func() {}), true},
+		{setField(struct{ _, a, _ func() }{}, 1*unsafe.Sizeof((func())(nil)), func() {}), false},
+		{setField(struct{ _, a, _ func() }{}, 2*unsafe.Sizeof((func())(nil)), func() {}), true},
 		// UnsafePointer
 		{(unsafe.Pointer)(nil), true},
 		{(unsafe.Pointer)(new(int)), false},

src/reflect/value.go

@@ -1594,10 +1594,9 @@ func (v Value) IsZero() bool {
 	case Uint, Uint8, Uint16, Uint32, Uint64, Uintptr:
 		return v.Uint() == 0
 	case Float32, Float64:
-		return math.Float64bits(v.Float()) == 0
+		return v.Float() == 0
 	case Complex64, Complex128:
-		c := v.Complex()
-		return math.Float64bits(real(c)) == 0 && math.Float64bits(imag(c)) == 0
+		return v.Complex() == 0
 	case Array:
 		// If the type is comparable, then compare directly with zero.
 		if v.typ().Equal != nil && v.typ().Size() <= maxZero {
@@ -1633,7 +1632,7 @@ func (v Value) IsZero() bool {
 
 		n := v.NumField()
 		for i := 0; i < n; i++ {
-			if !v.Field(i).IsZero() {
+			if !v.Field(i).IsZero() && v.Type().Field(i).Name != "_" {
 				return false
 			}
 		}