Allow scanning null even if PG and Go types are incompatible

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

pgtype/pgtype.go

@@ -558,6 +558,23 @@ type scanPlanFail struct {
 }
 
 func (plan *scanPlanFail) Scan(src []byte, dst any) error {
+	// If src is NULL it might be possible to scan into dst even though it is the types are not compatible. While this
+	// may seem to be a contrived case it can occur when selecting NULL directly. PostgreSQL assigns it the type of text.
+	// It would be surprising to the caller to have to cast the NULL (e.g. `select null::int`). So try to figure out a
+	// compatible data type for dst and scan with that.
+	//
+	// See https://github.com/jackc/pgx/issues/1326
+	if src == nil {
+		// As a horrible hack try all types to find anything that can scan into dst.
+		for oid := range plan.m.oidToType {
+			// using planScan instead of Scan or PlanScan to avoid polluting the planned scan cache.
+			plan := plan.m.planScan(oid, plan.formatCode, dst)
+			if _, ok := plan.(*scanPlanFail); !ok {
+				return plan.Scan(src, dst)
+			}
+		}
+	}
+
 	var format string
 	switch plan.formatCode {
 	case TextFormatCode:

pgtype/pgtype_test.go

@@ -329,6 +329,21 @@ func TestMapScanPointerToRenamedType(t *testing.T) {
 	assert.Equal(t, "foo", string(*rs))
 }
 
+// https://github.com/jackc/pgx/issues/1326
+func TestMapScanNullToWrongType(t *testing.T) {
+	m := pgtype.NewMap()
+
+	var n *int32
+	err := m.Scan(pgtype.TextOID, pgx.TextFormatCode, nil, &n)
+	assert.NoError(t, err)
+	assert.Nil(t, n)
+
+	var pn pgtype.Int4
+	err = m.Scan(pgtype.TextOID, pgx.TextFormatCode, nil, &pn)
+	assert.NoError(t, err)
+	assert.False(t, pn.Valid)
+}
+
 func BenchmarkMapScanInt4IntoBinaryDecoder(b *testing.B) {
 	m := pgtype.NewMap()
 	src := []byte{0, 0, 0, 42}