Correct field lookup in concept types

Fixes #6770

compiler/ccgexprs.nim

@@ -828,7 +828,7 @@ proc genCheckedRecordField(p: BProc, e: PNode, d: var TLoc) =
   if optFieldCheck in p.options:
     var a: TLoc
     genRecordFieldAux(p, e.sons[0], d, a)
-    let ty = skipTypes(a.t, abstractInst)
+    let ty = skipTypes(a.t, abstractInst + tyUserTypeClasses)
     var r = rdLoc(a)
     let f = e.sons[0].sons[1].sym
     let field = lookupFieldAgain(p, ty, f, r)

tests/concepts/t6770.nim

@@ -0,0 +1,27 @@
+discard """
+output: '''
+10
+10
+'''
+"""
+
+type GA = concept c
+  c.a is int
+
+type A = object
+  a: int
+
+type AA = object
+  case exists: bool
+  of true:
+    a: int
+  else:
+    discard
+
+proc print(inp: GA) =
+  echo inp.a
+
+let failing = AA(exists: true, a: 10)
+let working = A(a:10)
+print(working)
+print(failing)