diff --git a/compiler/seminst.nim b/compiler/seminst.nim
index abc5600c3a..f9a1377400 100644
--- a/compiler/seminst.nim
+++ b/compiler/seminst.nim
@@ -239,14 +239,20 @@ proc generateInstance(c: PContext, fn: PSym, pt: TIdTable,
   pushInfoContext(info)
   var entry = TInstantiation.new
   entry.sym = result
-  newSeq(entry.concreteTypes, gp.len)
+  # we need to compare both the generic types and the concrete types:
+  # generic[void](), generic[int]()
+  # see ttypeor.nim test.
   var i = 0
+  newSeq(entry.concreteTypes, fn.typ.len+gp.len)
   for s in instantiateGenericParamList(c, gp, pt):
     addDecl(c, s)
     entry.concreteTypes[i] = s.typ
     inc i
   pushProcCon(c, result)
   instantiateProcType(c, pt, result, info)
+  for j in 0 .. result.typ.len-1:
+    entry.concreteTypes[i] = result.typ.sons[j]
+    inc i
   if tfTriggersCompileTime in result.typ.flags:
     incl(result.flags, sfCompileTime)
   n.sons[genericParamsPos] = ast.emptyNode
diff --git a/compiler/semtypinst.nim b/compiler/semtypinst.nim
index f643fb9035..7957ac50a4 100644
--- a/compiler/semtypinst.nim
+++ b/compiler/semtypinst.nim
@@ -75,8 +75,12 @@ proc searchInstTypes*(key: PType): PType =
 proc cacheTypeInst*(inst: PType) =
   # XXX: add to module's generics
   #      update the refcount
-  let genericTyp = inst.sons[0]
-  genericTyp.sym.typeInstCache.safeAdd(inst)
+  let gt = inst.sons[0]
+  let t = if gt.kind == tyGenericBody: gt.lastSon else: gt
+  if t.kind in {tyStatic, tyGenericParam, tyIter} + tyTypeClasses:
+    return
+  gt.sym.typeInstCache.safeAdd(inst)
+
 
 type
   TReplTypeVars* {.final.} = object
diff --git a/compiler/types.nim b/compiler/types.nim
index 66fb657fc7..3846be8a0a 100644
--- a/compiler/types.nim
+++ b/compiler/types.nim
@@ -991,7 +991,9 @@ proc compareTypes*(x, y: PType,
   var c = initSameTypeClosure()
   c.cmp = cmp
   c.flags = flags
-  result = sameTypeAux(x, y, c)
+  if x == y: result = true
+  elif x.isNil or y.isNil: result = false
+  else: result = sameTypeAux(x, y, c)
 
 proc inheritanceDiff*(a, b: PType): int =
   # | returns: 0 iff `a` == `b`
diff --git a/tests/metatype/ttypeor.nim b/tests/metatype/ttypeor.nim
new file mode 100644
index 0000000000..f5acce7721
--- /dev/null
+++ b/tests/metatype/ttypeor.nim
@@ -0,0 +1,35 @@
+discard """
+  output: '''Foo
+Bar'''
+"""
+
+# bug #3338
+
+type
+  Base[T] = Foo[T] | Bar[T]
+
+  Foo[T] = ref object
+    x: T
+
+  Bar[T] = ref object
+    x: T
+
+proc test[T](ks: Foo[T], x, y: T): T =
+  echo("Foo")
+  return x + y + ks.x
+
+proc test[T](ks: Bar[T], x, y: T): T =
+  echo("Bar")
+  return x
+
+proc add[T](ksa: Base[T]) =
+  var test = ksa.test(5, 10)
+  ksa.x = test
+
+var t1 = Foo[int32]()
+t1.add()
+doAssert t1.x == 15
+
+var t2 = Bar[int32]()
+t2.add()
+doAssert t2.x == 5