fix #5968

compiler/ccgexprs.nim

@@ -665,7 +665,9 @@ proc genDeref(p: BProc, e: PNode, d: var TLoc; enforceDeref=false) =
       d.s = OnHeap
   else:
     var a: TLoc
-    let typ = skipTypes(e.sons[0].typ, abstractInst)
+    var typ = skipTypes(e.sons[0].typ, abstractInst)
+    if typ.kind in {tyUserTypeClass, tyUserTypeClassInst} and typ.isResolvedUserTypeClass:
+      typ = typ.lastSon
     if typ.kind == tyVar and tfVarIsPtr notin typ.flags and p.module.compileToCpp and e.sons[0].kind == nkHiddenAddr:
       initLocExprSingleUse(p, e[0][0], d)
       return

tests/concepts/t5968.nim

@@ -0,0 +1,20 @@
+discard """
+  exitcode: 0
+"""
+
+type
+  Enumerable[T] = concept e
+    for it in e:
+      it is T
+
+proc cmap[T, G](e: Enumerable[T], fn: proc(t: T): G): seq[G] =
+  result = @[]
+  for it in e: result.add(fn(it))
+
+import json
+
+var x = %["hello", "world"]
+
+var z = x.cmap(proc(it: JsonNode): string = it.getStr & "!")
+assert z == @["hello!", "world!"]
+