[Solved]-Scala case class copy doesn't always work with `_` existential type-scala

After the compiler handles named and default parameters, the calls become

foo.copy("foo1.1", foo.name, foo.v1)

and

foo.copy("foo1.1", foo.name, foo.v1, foo.v2)

respectively. Or, if you replace the parameters with types,

foo.copy[?](String, String, Bar[_])

and

foo.copy[?](String, String, Bar[_], Bar[_])

? is the type parameter of copy which has to inferred. In the first case the compiler basically says "? is the type parameter of Bar[_], even if I don't know what that is".

In the second case the type parameters of two Bar[_] must really be the same, but that information is lost by the time the compiler is inferring ?; they are just Bar[_], and not something like Bar[foo's unknown type parameter]. So e.g. "? is the type parameter of first Bar[_], even if I don't know what that is" won't work because so far as the compiler knows, the second Bar[_] could be different.

It isn't a bug in the sense that it follows the language specification; and changing the specification to allow this would take significant effort and make both it and the compiler more complicated. It may not be a good trade-off for such a relatively rare case.

Another workaround is to use type variable pattern to temporarily give a name to _:

foo match { case foo: Foo[a] => foo.copy(id = "foo1.1") }

The compiler now sees that foo.v1 and foo.v2 are both Bar[a] and so the result of copy is Foo[a]. After leaving the case branch it becomes Foo[_].