fixes #3305; error message will improve with later improvements for concepts

compiler/sigmatch.nim

@@ -511,6 +511,9 @@ proc typeRangeRel(f, a: PType): TTypeRelation {.noinline.} =
 proc matchUserTypeClass*(c: PContext, m: var TCandidate,
                          ff, a: PType): TTypeRelation =
   var body = ff.skipTypes({tyUserTypeClassInst})
+  if c.inTypeClass > 20:
+    localError(body.n[3].info, $body.n[3] & " too nested for type matching")
+    return isNone
 
   openScope(c)
   inc c.inTypeClass