fixes #3686

compiler/sigmatch.nim

@@ -1301,11 +1301,8 @@ proc paramTypesMatchAux(m: var TCandidate, f, argType: PType,
         arg.typ.n = evaluated
         argType = arg.typ
 
-  var
+  var a = argType
-    a = if c.inTypeClass > 0: argType.skipTypes({tyTypeDesc, tyFieldAccessor})
+  var r = typeRel(m, f, a)
-        else: argType
-
-    r = typeRel(m, f, a)
 
   if r != isNone and m.calleeSym != nil and
      m.calleeSym.kind in {skMacro, skTemplate}:

tests/concepts/tmonoid.nim

@@ -0,0 +1,13 @@
+discard """
+  output: '''true'''
+"""
+
+# bug #3686
+
+type Monoid = concept x, y
+  x + y is type(x)
+  type(z(type(x))) is type(x)
+
+proc z(x: typedesc[int]): int = 0
+
+echo(int is Monoid)