Support sql.Scanner on renamed base type

https://github.com/jackc/pgtype/issues/197

pgtype/pgtype.go

@@ -1227,6 +1227,16 @@ func (m *Map) planScan(oid uint32, formatCode int16, target any) ScanPlan {
 		}
 	}
 
+	// This needs to happen before trying m.TryWrapScanPlanFuncs. Otherwise, a sql.Scanner would not get called if it was
+	// defined on a type that could be unwrapped such as `type myString string`.
+	//
+	//  https://github.com/jackc/pgtype/issues/197
+	if dt == nil {
+		if _, ok := target.(sql.Scanner); ok {
+			return &scanPlanSQLScanner{formatCode: formatCode}
+		}
+	}
+
 	for _, f := range m.TryWrapScanPlanFuncs {
 		if wrapperPlan, nextDst, ok := f(target); ok {
 			if nextPlan := m.planScan(oid, formatCode, nextDst); nextPlan != nil {
@@ -1248,10 +1258,6 @@ func (m *Map) planScan(oid uint32, formatCode int16, target any) ScanPlan {
 		}
 	}
 
-	if _, ok := target.(sql.Scanner); ok {
-		return &scanPlanSQLScanner{formatCode: formatCode}
-	}
-
 	return &scanPlanFail{m: m, oid: oid, formatCode: formatCode}
 }
 

pgtype/pgtype_test.go

@@ -223,6 +223,23 @@ func TestMapScanUnknownOIDIntoSQLScanner(t *testing.T) {
 	assert.False(t, s.Valid)
 }
 
+type scannerString string
+
+func (ss *scannerString) Scan(v any) error {
+	*ss = scannerString("scanned")
+	return nil
+}
+
+// https://github.com/jackc/pgtype/issues/197
+func TestMapScanUnregisteredOIDIntoRenamedStringSQLScanner(t *testing.T) {
+	m := pgtype.NewMap()
+
+	var s scannerString
+	err := m.Scan(unregisteredOID, pgx.TextFormatCode, []byte(nil), &s)
+	assert.NoError(t, err)
+	assert.Equal(t, "scanned", string(s))
+}
+
 type pgCustomInt int64
 
 func (ci *pgCustomInt) Scan(src interface{}) error {