reflect: add TypeAssert

Author: jakebailey

src/internal/reflectlite/value.go

@@ -86,6 +86,64 @@ func (v Value) Interface() interface{} {
 	return valueInterfaceUnsafe(v)
 }
 
+func TypeAssert[T any](v Value) (T, bool) {
+	if v.typecode == nil {
+		panic("reflect.TypeAssert: zero Value")
+	}
+	if !v.isExported() {
+		// Do not allow access to unexported values via TypeAssert,
+		// because they might be pointers that should not be
+		// writable or methods or function that should not be callable.
+		panic("reflect.TypeAssert: cannot return value obtained from unexported field or method")
+	}
+
+	typ := TypeFor[T]()
+
+	// If v is an interface, return the element inside the interface.
+	//
+	// T is a concrete type and v is an interface. For example:
+	//
+	//	var v any = int(1)
+	//	val := ValueOf(&v).Elem()
+	//	TypeAssert[int](val) == val.Interface().(int)
+	//
+	// T is a interface and v is a non-nil interface value. For example:
+	//
+	//	var v any = &someError{}
+	//	val := ValueOf(&v).Elem()
+	//	TypeAssert[error](val) == val.Interface().(error)
+	//
+	// T is a interface and v is a nil interface value. For example:
+	//
+	//	var v error = nil
+	//	val := ValueOf(&v).Elem()
+	//	TypeAssert[error](val) == val.Interface().(error)
+	if v.Kind() == Interface {
+		val, ok := valueInterfaceUnsafe(v).(T)
+		return val, ok
+	}
+
+	// If T is an interface and v is a concrete type. For example:
+	//
+	//	TypeAssert[any](ValueOf(1)) == ValueOf(1).Interface().(any)
+	//	TypeAssert[error](ValueOf(&someError{})) == ValueOf(&someError{}).Interface().(error)
+	if typ.Kind() == Interface {
+		val, ok := valueInterfaceUnsafe(v).(T)
+		return val, ok
+	}
+
+	// Both v and T must be concrete types.
+	// The only way for an type-assertion to match is if the types are equal.
+	if typ != v.typecode {
+		var zero T
+		return zero, false
+	}
+	if !v.isIndirect() {
+		return *(*T)(unsafe.Pointer(&v.value)), true
+	}
+	return *(*T)(v.value), true
+}
+
 // valueInterfaceUnsafe is used by the runtime to hash map keys. It should not
 // be subject to the isExported check.
 func valueInterfaceUnsafe(v Value) interface{} {

src/reflect/value.go

@@ -17,6 +17,10 @@ func ValueOf(i interface{}) Value {
 	return Value{reflectlite.ValueOf(i)}
 }
 
+func TypeAssert[T any](v Value) (T, bool) {
+	return reflectlite.TypeAssert[T](v.Value)
+}
+
 func (v Value) Type() Type {
 	return toType(v.Value.Type())
 }

src/reflect/value_test.go

@@ -3,6 +3,7 @@ package reflect_test
 import (
 	"bytes"
 	"encoding/base64"
+	"fmt"
 	. "reflect"
 	"slices"
 	"sort"
@@ -869,3 +870,73 @@ func equal[T comparable](a, b []T) bool {
 	}
 	return true
 }
+
+func TestTypeAssert(t *testing.T) {
+	testTypeAssert(t, int(123456789), int(123456789), true)
+	testTypeAssert(t, int(-123456789), int(-123456789), true)
+	testTypeAssert(t, int32(123456789), int32(123456789), true)
+	testTypeAssert(t, int8(-123), int8(-123), true)
+	testTypeAssert(t, [2]int{1234, -5678}, [2]int{1234, -5678}, true)
+	testTypeAssert(t, "test value", "test value", true)
+	testTypeAssert(t, any("test value"), any("test value"), true)
+
+	v := 123456789
+	testTypeAssert(t, &v, &v, true)
+
+	testTypeAssert(t, int(123), uint(0), false)
+
+	testTypeAssert[any](t, 1, 1, true)
+	testTypeAssert[fmt.Stringer](t, 1, nil, false)
+
+	vv := testTypeWithMethod{"test"}
+	testTypeAssert[any](t, vv, vv, true)
+	testTypeAssert[any](t, &vv, &vv, true)
+	testTypeAssert[fmt.Stringer](t, vv, vv, true)
+	testTypeAssert[fmt.Stringer](t, &vv, &vv, true)
+	testTypeAssert[interface{ A() }](t, vv, nil, false)
+	testTypeAssert[interface{ A() }](t, &vv, nil, false)
+	testTypeAssert(t, any(vv), any(vv), true)
+	testTypeAssert(t, fmt.Stringer(vv), fmt.Stringer(vv), true)
+
+	testTypeAssert(t, fmt.Stringer(vv), any(vv), true)
+	testTypeAssert(t, any(vv), fmt.Stringer(vv), true)
+	testTypeAssert(t, fmt.Stringer(vv), interface{ M() }(vv), true)
+	testTypeAssert(t, interface{ M() }(vv), fmt.Stringer(vv), true)
+
+	testTypeAssert(t, any(int(1)), int(1), true)
+	testTypeAssert(t, any(int(1)), byte(0), false)
+	testTypeAssert(t, fmt.Stringer(vv), vv, true)
+}
+
+func testTypeAssert[T comparable, V any](t *testing.T, val V, wantVal T, wantOk bool) {
+	t.Helper()
+
+	v, ok := TypeAssert[T](ValueOf(&val).Elem())
+	if v != wantVal || ok != wantOk {
+		t.Errorf("TypeAssert[%v](%#v) = (%#v, %v); want = (%#v, %v)", TypeFor[T](), val, v, ok, wantVal, wantOk)
+	}
+
+	// Additionally make sure that TypeAssert[T](v) behaves in the same way as v.Interface().(T).
+	v2, ok2 := ValueOf(&val).Elem().Interface().(T)
+	if v != v2 || ok != ok2 {
+		t.Errorf("reflect.ValueOf(%#v).Interface().(%v) = (%#v, %v); want = (%#v, %v)", val, TypeFor[T](), v2, ok2, v, ok)
+	}
+}
+
+type testTypeWithMethod struct{ val string }
+
+func (v testTypeWithMethod) String() string { return v.val }
+func (v testTypeWithMethod) M()             {}
+
+func TestTypeAssertPanic(t *testing.T) {
+	t.Run("zero val", func(t *testing.T) {
+		defer func() { recover() }()
+		TypeAssert[int](Value{})
+		t.Fatalf("TypeAssert did not panic")
+	})
+	t.Run("read only", func(t *testing.T) {
+		defer func() { recover() }()
+		TypeAssert[int](ValueOf(&testTypeWithMethod{}).FieldByName("val"))
+		t.Fatalf("TypeAssert did not panic")
+	})
+}