fix #5642

compiler/ccgexprs.nim

@@ -756,7 +756,7 @@ proc genRecordField(p: BProc, e: PNode, d: var TLoc) =
   genRecordFieldAux(p, e, d, a)
   var r = rdLoc(a)
   var f = e.sons[1].sym
-  let ty = skipTypes(a.t, abstractInst)
+  let ty = skipTypes(a.t, abstractInst + tyUserTypeClasses)
   if ty.kind == tyTuple:
     # we found a unique tuple type which lacks field information
     # so we use Field$i

compiler/semexprs.nim

@@ -1155,6 +1155,8 @@ proc builtinFieldAccess(c: PContext, n: PNode, flags: TExprFlags): PNode =
     ty = n.sons[0].typ
     return nil
   ty = skipTypes(ty, {tyGenericInst, tyVar, tyPtr, tyRef, tyAlias})
+  if ty.kind in tyUserTypeClasses and ty.isResolvedUserTypeClass:
+    ty = ty.lastSon
   while tfBorrowDot in ty.flags: ty = ty.skipTypes({tyDistinct})
   var check: PNode = nil
   if ty.kind == tyObject:

compiler/semfields.nim

@@ -121,12 +121,13 @@ proc semForFields(c: PContext, n: PNode, m: TMagic): PNode =
     localError(n.info, errWrongNumberOfVariables)
     return result
 
-  var tupleTypeA = skipTypes(call.sons[1].typ, abstractVar-{tyTypeDesc})
+  const skippedTypesForFields = abstractVar - {tyTypeDesc} + tyUserTypeClasses
+  var tupleTypeA = skipTypes(call.sons[1].typ, skippedTypesForFields)
   if tupleTypeA.kind notin {tyTuple, tyObject}:
     localError(n.info, errGenerated, "no object or tuple type")
     return result
   for i in 1..call.len-1:
-    var tupleTypeB = skipTypes(call.sons[i].typ, abstractVar-{tyTypeDesc})
+    var tupleTypeB = skipTypes(call.sons[i].typ, skippedTypesForFields)
     if not sameType(tupleTypeA, tupleTypeB):
       typeMismatch(call.sons[i].info, tupleTypeA, tupleTypeB)
 

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)
+