Move typed nil handling to Map.Encode from anynil

The new logic checks for any type of nil at the beginning of Encode and
then either treats it as NULL or calls the driver.Valuer method if
appropriate.

This should preserve the existing nil normalization while restoring the
ability to encode nil driver.Valuer values.

pgtype/array_codec.go

@@ -6,7 +6,6 @@ import (
 	"fmt"
 	"reflect"
 
-	"github.com/jackc/pgx/v5/internal/anynil"
 	"github.com/jackc/pgx/v5/internal/pgio"
 )
 
@@ -230,7 +229,7 @@ func (c *ArrayCodec) PlanScan(m *Map, oid uint32, format int16, target any) Scan
 
 	// target / arrayScanner might be a pointer to a nil. If it is create one so we can call ScanIndexType to plan the
 	// scan of the elements.
-	if anynil.Is(target) {
+	if isNil, _ := isNilDriverValuer(target); isNil {
 		arrayScanner = reflect.New(reflect.TypeOf(target).Elem()).Interface().(ArraySetter)
 	}
 

pgtype/doc.go

@@ -141,11 +141,11 @@ 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).
+pgtype encodes untyped and typed nils (e.g. nil and []byte(nil)) to the SQL NULL value without going through the Codec
+system. This means that Codecs and other encoding logic do not have to handle nil or *T(nil).
 
 However, database/sql compatibility requires Value to be called on T(nil) when T implements driver.Valuer. Therefore,
-driver.Valuer values are not normalized to nil unless it is a *T(nil) where driver.Valuer is implemented on T. See
+driver.Valuer values are only considered NULL when *T(nil) where driver.Valuer is implemented on T not on *T. See
 https://github.com/golang/go/issues/8415 and
 https://github.com/golang/go/commit/0ce1d79a6a771f7449ec493b993ed2a720917870.
 

pgtype/pgtype.go

@@ -9,8 +9,6 @@ import (
 	"net/netip"
 	"reflect"
 	"time"
-
-	"github.com/jackc/pgx/v5/internal/anynil"
 )
 
 // PostgreSQL oids for common types
@@ -1914,8 +1912,17 @@ func newEncodeError(value any, m *Map, oid uint32, formatCode int16, err error)
 // (nil, nil). The caller of Encode is responsible for writing the correct NULL value or the length of the data
 // written.
 func (m *Map) Encode(oid uint32, formatCode int16, value any, buf []byte) (newBuf []byte, err error) {
-	if anynil.Is(value) {
-		return nil, nil
+	if isNil, callNilDriverValuer := isNilDriverValuer(value); isNil {
+		if callNilDriverValuer {
+			newBuf, err = (&encodePlanDriverValuer{m: m, oid: oid, formatCode: formatCode}).Encode(value, buf)
+			if err != nil {
+				return nil, newEncodeError(value, m, oid, formatCode, err)
+			}
+
+			return newBuf, nil
+		} else {
+			return nil, nil
+		}
 	}
 
 	plan := m.PlanEncode(oid, formatCode, value)
@@ -1970,3 +1977,55 @@ func (w *sqlScannerWrapper) Scan(src any) error {
 
 	return w.m.Scan(t.OID, TextFormatCode, bufSrc, w.v)
 }
+
+// canBeNil returns true if value can be nil.
+func canBeNil(value any) bool {
+	refVal := reflect.ValueOf(value)
+	kind := refVal.Kind()
+	switch kind {
+	case reflect.Chan, reflect.Func, reflect.Map, reflect.Ptr, reflect.UnsafePointer, reflect.Interface, reflect.Slice:
+		return true
+	default:
+		return false
+	}
+}
+
+// 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()
+
+// isNilDriverValuer returns true if value is any type of nil unless it implements driver.Valuer. *T is not considered to implement
+// driver.Valuer if it is only implemented by T.
+func isNilDriverValuer(value any) (isNil bool, callNilDriverValuer bool) {
+	if value == nil {
+		return true, false
+	}
+
+	refVal := reflect.ValueOf(value)
+	kind := refVal.Kind()
+	switch kind {
+	case reflect.Chan, reflect.Func, reflect.Map, reflect.Ptr, reflect.UnsafePointer, reflect.Interface, reflect.Slice:
+		if !refVal.IsNil() {
+			return false, false
+		}
+
+		if _, ok := value.(driver.Valuer); ok {
+			if kind == reflect.Ptr {
+				// The type assertion will succeed if driver.Valuer is implemented on T or *T. Check if it is implemented on *T
+				// by checking if it is not implemented on *T.
+				return true, !refVal.Type().Elem().Implements(valuerReflectType)
+			} else {
+				return true, true
+			}
+		}
+
+		return true, false
+	default:
+		return false, false
+	}
+}