fix #5642

tests/concepts/t5642.nim

@@ -0,0 +1,25 @@
+discard """
+  output: 9
+"""
+
+type DataTable = concept x
+  x is object
+  for f in fields(x):
+    f is seq
+
+type Students = object
+   id : seq[int]
+   name : seq[string]
+   age: seq[int]
+
+proc nrow*(dt: DataTable) : Natural =
+  var totalLen = 0
+  for f in fields(dt):
+    totalLen += f.len
+  return totalLen
+
+let
+ stud = Students (id : @[1,2,3], name : @["Vas", "Pas", "NafNaf"], age : @[10,16,32])
+
+echo nrow(stud)
+