Fix encode driver.Valuer on pointer

internal/anynil/anynil.go

@@ -1,17 +1,47 @@
 package anynil
 
-import "reflect"
+import (
+	"database/sql/driver"
+	"reflect"
+)
 
-// Is returns true if value is any type of nil. e.g. nil or []byte(nil).
+// valuerReflectType is a reflect.Type for driver.Valuer. It has confusing syntax because reflect.TypeOf returns nil
+// when it's argument is a nil interface value. So we use a pointer to the interface and call Elem to get the actual
+// type. Yuck.
+//
+// This can be simplified in Go 1.22 with reflect.TypeFor.
+//
+// var valuerReflectType = reflect.TypeFor[driver.Valuer]()
+var valuerReflectType = reflect.TypeOf((*driver.Valuer)(nil)).Elem()
+
+// Is returns true if value is any type of nil except a pointer that directly implements driver.Valuer. e.g. nil,
+// []byte(nil), and a *T where T implements driver.Valuer get normalized to nil but a *T where *T implements
+// driver.Valuer does not.
 func Is(value any) bool {
 	if value == nil {
 		return true
 	}
 
 	refVal := reflect.ValueOf(value)
-	switch refVal.Kind() {
+	kind := refVal.Kind()
+	switch kind {
 	case reflect.Chan, reflect.Func, reflect.Map, reflect.Ptr, reflect.UnsafePointer, reflect.Interface, reflect.Slice:
-		return refVal.IsNil()
+		if !refVal.IsNil() {
+			return false
+		}
+
+		if kind == reflect.Ptr {
+			if _, ok := value.(driver.Valuer); ok {
+				// The pointer will be considered to implement driver.Valuer even if it is actually implemented on the value.
+				// But we only want to consider it nil if it is implemented on the pointer. So check if what the pointer points
+				// to implements driver.Valuer.
+				if !refVal.Type().Elem().Implements(valuerReflectType) {
+					return false
+				}
+			}
+		}
+
+		return true
 	default:
 		return false
 	}

pgtype/doc.go

@@ -139,6 +139,16 @@ Compatibility with database/sql
 pgtype also includes support for custom types implementing the database/sql.Scanner and database/sql/driver.Valuer
 interfaces.
 
+Encoding Typed Nils
+
+pgtype normalizes typed nils (e.g. []byte(nil)) into nil. nil is always encoded is the SQL NULL value without going
+through the Codec system. This means that Codecs and other encoding logic does not have to handle nil or *T(nil).
+
+However, database/sql compatibility requires Value to be called on a pointer that implements driver.Valuer. See
+https://github.com/golang/go/issues/8415 and
+https://github.com/golang/go/commit/0ce1d79a6a771f7449ec493b993ed2a720917870. Therefore, pointers that implement
+driver.Valuer are not normalized to nil.
+
 Child Records
 
 pgtype's support for arrays and composite records can be used to load records and their children in a single query.  See

query_test.go

@@ -4,6 +4,8 @@ import (
 	"bytes"
 	"context"
 	"database/sql"
+	"database/sql/driver"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"os"
@@ -1171,6 +1173,54 @@ func TestConnQueryDatabaseSQLDriverValuerWithAutoGeneratedPointerReceiver(t *tes
 	ensureConnValid(t, conn)
 }
 
+type nilAsEmptyJSONObject struct {
+	ID   string
+	Name string
+}
+
+func (v *nilAsEmptyJSONObject) Value() (driver.Value, error) {
+	if v == nil {
+		return "{}", nil
+	}
+
+	return json.Marshal(v)
+}
+
+// https://github.com/jackc/pgx/issues/1566
+func TestConnQueryDatabaseSQLDriverValuerCalledOnPointerImplementers(t *testing.T) {
+	t.Parallel()
+
+	conn := mustConnectString(t, os.Getenv("PGX_TEST_DATABASE"))
+	defer closeConn(t, conn)
+
+	mustExec(t, conn, "create temporary table t(v json not null)")
+
+	var v *nilAsEmptyJSONObject
+	commandTag, err := conn.Exec(context.Background(), `insert into t(v) values($1)`, v)
+	require.NoError(t, err)
+	require.Equal(t, "INSERT 0 1", commandTag.String())
+
+	var s string
+	err = conn.QueryRow(context.Background(), "select v from t").Scan(&s)
+	require.NoError(t, err)
+	require.Equal(t, "{}", s)
+
+	_, err = conn.Exec(context.Background(), `delete from t`)
+	require.NoError(t, err)
+
+	v = &nilAsEmptyJSONObject{ID: "1", Name: "foo"}
+	commandTag, err = conn.Exec(context.Background(), `insert into t(v) values($1)`, v)
+	require.NoError(t, err)
+	require.Equal(t, "INSERT 0 1", commandTag.String())
+
+	var v2 *nilAsEmptyJSONObject
+	err = conn.QueryRow(context.Background(), "select v from t").Scan(&v2)
+	require.NoError(t, err)
+	require.Equal(t, v, v2)
+
+	ensureConnValid(t, conn)
+}
+
 func TestConnQueryDatabaseSQLDriverScannerWithBinaryPgTypeThatAcceptsSameType(t *testing.T) {
 	t.Parallel()