Fix scan to pointer to pointer to renamed type

refs https://github.com/jackc/pgx/issues/1326

pgtype/pgtype.go

@@ -659,7 +659,12 @@ func TryFindUnderlyingTypeScanPlan(dst any) (plan WrappedScanPlanNextSetter, nex
 	dstValue := reflect.ValueOf(dst)
 
 	if dstValue.Kind() == reflect.Ptr {
-		elemValue := dstValue.Elem()
+		var elemValue reflect.Value
+		if dstValue.IsNil() {
+			elemValue = reflect.New(dstValue.Type().Elem()).Elem()
+		} else {
+			elemValue = dstValue.Elem()
+		}
 		nextDstType := elemKindToPointerTypes[elemValue.Kind()]
 		if nextDstType == nil && elemValue.Kind() == reflect.Slice {
 			if elemValue.Type().Elem().Kind() == reflect.Uint8 {

pgtype/pgtype_test.go

@@ -317,6 +317,18 @@ func TestMapEncodeBinaryFormatDatabaseValuerThatReturnsString(t *testing.T) {
 	require.Equal(t, []byte{0, 0, 0, 42}, buf)
 }
 
+// https://github.com/jackc/pgx/issues/1326
+func TestMapScanPointerToRenamedType(t *testing.T) {
+	srcBuf := []byte("foo")
+	m := pgtype.NewMap()
+
+	var rs *_string
+	err := m.Scan(pgtype.TextOID, pgx.TextFormatCode, srcBuf, &rs)
+	assert.NoError(t, err)
+	require.NotNil(t, rs)
+	assert.Equal(t, "foo", string(*rs))
+}
+
 func BenchmarkMapScanInt4IntoBinaryDecoder(b *testing.B) {
 	m := pgtype.NewMap()
 	src := []byte{0, 0, 0, 42}