fixes #18095 (#25744)

compiler/semtypes.nim

@@ -2380,7 +2380,7 @@ proc semTypeNode(c: PContext, n: PNode, prev: PType): PType =
     else:
       result = typeExpr.typ.base
       if result.isMetaType and
-         result.kind != tyUserTypeClass:
+         result.kind notin tyTypeClasses:
            # the dot expression may refer to a concept type in
            # a different module. allow a normal alias then.
         let preprocessed = semGenericStmt(c, n)

tests/metatype/deps/cisaorb.nim

@@ -0,0 +1,8 @@
+type
+  A* = object
+    discard
+
+  B* = object
+    discard
+
+  C* = A | B

tests/metatype/tcisaorb.nim

@@ -0,0 +1,22 @@
+discard """
+  action: "compile"
+"""
+import deps/cisaorb
+
+when true:
+  # These work fine.
+  discard default(cisaorb.A)
+  proc f1(x: cisaorb.A) = discard
+  discard default(cisaorb.B)
+  proc f2(x: cisaorb.B) = discard
+  discard default(A)
+  proc f3(x: A) = discard
+  discard default(B)
+  proc f4(x: B) = discard
+  proc f5(x: C) = discard
+  proc f6(x: cisaorb.C | C) = discard
+
+proc doesWork(x: A | B) = discard
+
+# Doesn't compile.
+proc f(x: cisaorb.C) = discard