Hello,
@startuml
class Foo< T >
class "Foo<int>" as Foo_int
class "Foo<double>" as Foo_double
@enduml
But there is a bug when you try:
@startuml
class Foo< T > as Foo_t
class "Foo<int>" as Foo_int
class "Foo<double>" as Foo_double
Foo_t <|-- Foo_int
Foo_t <|-- Foo_double
@enduml