reflect: Add Value.IsZero() method

Add Value.IsZero() with tests, largely copied from the Go source code.

The tests were altered to remove the parts calling `Zero()` as that is
still unimplemented in tinygo, and to remove a test that tries to catch
a panic which is not supported on wasi.

A new case for `UnsafePointer` in `Value.IsNil()` was required for
unsafe.Pointer tests to pass.

Link: https://cs.opensource.google/go/go/+/refs/tags/go1.19.3:src/reflect/value.go;l=1568
Link: https://cs.opensource.google/go/go/+/refs/tags/go1.19.3:src/reflect/all_test.go;l=1322
Co-authored-by: Cam Hutchison <camh@xdna.net>

src/reflect/all_test.go

@@ -5,9 +5,13 @@
 package reflect_test
 
 import (
+	"io"
 	"math"
 	. "reflect"
+	"strings"
 	"testing"
+	"time"
+	"unsafe"
 )
 
 type Basic struct {
@@ -184,5 +188,107 @@ func TestDeepEqualComplexStructInequality(t *testing.T) {
 	}
 }
 
+type T struct {
+	a int
+	b float64
+	c string
+	d *int
+}
+
+var _i = 7
+
+func TestIsZero(t *testing.T) {
+	for i, tt := range []struct {
+		x    interface{}
+		want bool
+	}{
+		// Booleans
+		{true, false},
+		{false, true},
+		// Numeric types
+		{int(0), true},
+		{int(1), false},
+		{int8(0), true},
+		{int8(1), false},
+		{int16(0), true},
+		{int16(1), false},
+		{int32(0), true},
+		{int32(1), false},
+		{int64(0), true},
+		{int64(1), false},
+		{uint(0), true},
+		{uint(1), false},
+		{uint8(0), true},
+		{uint8(1), false},
+		{uint16(0), true},
+		{uint16(1), false},
+		{uint32(0), true},
+		{uint32(1), false},
+		{uint64(0), true},
+		{uint64(1), false},
+		{float32(0), true},
+		{float32(1.2), false},
+		{float64(0), true},
+		{float64(1.2), false},
+		{math.Copysign(0, -1), false},
+		{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},
+		{uintptr(0), true},
+		{uintptr(128), false},
+		// Array
+		{[5]string{"", "", "", "", ""}, true},
+		{[5]string{}, true},
+		{[5]string{"", "", "", "a", ""}, false},
+		// Chan
+		{(chan string)(nil), true},
+		{make(chan string), false},
+		{time.After(1), false},
+		// Func
+		{(func())(nil), true},
+		{New, false},
+		// Interface
+		{New(TypeOf(new(error)).Elem()).Elem(), true},
+		{(io.Reader)(strings.NewReader("")), false},
+		// Map
+		{(map[string]string)(nil), true},
+		{map[string]string{}, false},
+		{make(map[string]string), false},
+		// Pointer
+		{(*func())(nil), true},
+		{(*int)(nil), true},
+		{new(int), false},
+		// Slice
+		{[]string{}, false},
+		{([]string)(nil), true},
+		{make([]string, 0), false},
+		// Strings
+		{"", true},
+		{"not-zero", false},
+		// Structs
+		{T{}, true},
+		{T{123, 456.75, "hello", &_i}, false},
+		// UnsafePointer
+		{(unsafe.Pointer)(nil), true},
+		{(unsafe.Pointer)(new(int)), false},
+	} {
+		var x Value
+		if v, ok := tt.x.(Value); ok {
+			x = v
+		} else {
+			x = ValueOf(tt.x)
+		}
+
+		b := x.IsZero()
+		if b != tt.want {
+			t.Errorf("%d: IsZero((%s)(%+v)) = %t, want %t", i, x.Kind(), tt.x, b, tt.want)
+		}
+	}
+}
+
 type MyBytes []byte
 type MyByte byte

src/reflect/value.go

@@ -1,6 +1,7 @@
 package reflect
 
 import (
+	"math"
 	"unsafe"
 )
 
@@ -91,6 +92,46 @@ func (v Value) Type() Type {
 	return v.typecode
 }
 
+// IsZero reports whether v is the zero value for its type.
+// It panics if the argument is invalid.
+func (v Value) IsZero() bool {
+	switch v.Kind() {
+	case Bool:
+		return !v.Bool()
+	case Int, Int8, Int16, Int32, Int64:
+		return v.Int() == 0
+	case Uint, Uint8, Uint16, Uint32, Uint64, Uintptr:
+		return v.Uint() == 0
+	case Float32, Float64:
+		return math.Float64bits(v.Float()) == 0
+	case Complex64, Complex128:
+		c := v.Complex()
+		return math.Float64bits(real(c)) == 0 && math.Float64bits(imag(c)) == 0
+	case Array:
+		for i := 0; i < v.Len(); i++ {
+			if !v.Index(i).IsZero() {
+				return false
+			}
+		}
+		return true
+	case Chan, Func, Interface, Map, Pointer, Slice, UnsafePointer:
+		return v.IsNil()
+	case String:
+		return v.Len() == 0
+	case Struct:
+		for i := 0; i < v.NumField(); i++ {
+			if !v.Field(i).IsZero() {
+				return false
+			}
+		}
+		return true
+	default:
+		// This should never happens, but will act as a safeguard for
+		// later, as a default value doesn't makes sense here.
+		panic(&ValueError{"reflect.Value.IsZero", v.Kind()})
+	}
+}
+
 // Internal function only, do not use.
 //
 // RawType returns the raw, underlying type code. It is used in the runtime
@@ -107,7 +148,7 @@ func (v Value) Kind() Kind {
 // is not a channel, map, pointer, function, slice, or interface.
 func (v Value) IsNil() bool {
 	switch v.Kind() {
-	case Chan, Map, Ptr:
+	case Chan, Map, Ptr, UnsafePointer:
 		return v.pointer() == nil
 	case Func:
 		if v.value == nil {