fixes #16897 [backport:1.2] (#16900)

Araq · web-flow · commit 91ace2188a24 · 2021-02-01T16:56:02.000+01:00
diff --git a/compiler/types.nim b/compiler/types.nim
@@ -73,7 +73,7 @@ const
                tyInferred, tySink, tyLent, tyOwned}
   # typedescX is used if we're sure tyTypeDesc should be included (or skipped)
   typedescPtrs* = abstractPtrs + {tyTypeDesc}
-  typedescInst* = abstractInst + {tyTypeDesc, tyOwned}
+  typedescInst* = abstractInst + {tyTypeDesc, tyOwned, tyUserTypeClass}
 
 proc invalidGenericInst*(f: PType): bool =
   result = f.kind == tyGenericInst and lastSon(f) == nil
