Fix scan pointer to pointer to nil slice

https://github.com/jackc/pgx/issues/1263

pgtype/array_codec.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"reflect"
 
+	"github.com/jackc/pgx/v5/internal/anynil"
 	"github.com/jackc/pgx/v5/internal/pgio"
 )
 
@@ -216,6 +217,12 @@ func (c *ArrayCodec) PlanScan(m *Map, oid uint32, format int16, target any) Scan
 		return nil
 	}
 
+	// 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) {
+		arrayScanner = reflect.New(reflect.TypeOf(target).Elem()).Interface().(ArraySetter)
+	}
+
 	elementType := arrayScanner.ScanIndexType()
 
 	elementScanPlan := m.PlanScan(c.ElementType.OID, format, elementType)

pgtype/pgtype_test.go

@@ -254,6 +254,17 @@ func TestScanPlanInterface(t *testing.T) {
 	assert.Error(t, err)
 }
 
+// https://github.com/jackc/pgx/issues/1263
+func TestMapScanPtrToPtrToSlice(t *testing.T) {
+	m := pgtype.NewMap()
+	src := []byte("{foo,bar}")
+	var v *[]string
+	plan := m.PlanScan(pgtype.TextArrayOID, pgtype.TextFormatCode, &v)
+	err := plan.Scan(src, &v)
+	require.NoError(t, err)
+	require.Equal(t, []string{"foo", "bar"}, *v)
+}
+
 func BenchmarkTypeMapScanInt4IntoBinaryDecoder(b *testing.B) {
 	m := pgtype.NewMap()
 	src := []byte{0, 0, 0, 42}