Type bound error when creating a tuple in scala

Question

Say I have a type

trait Mode[T]
trait MyType[T, M <: Mode[T]]

This compiles

val t: MyType[_, _] = ???
t

But not this one

val t: MyType[_, _] = ???
"some_string" -> t

Error says something like type arguments [_$0,_$1] do not conform to trait MyType's type parameter bounds

So my question is that why this does not compile on tuple creation?

Answer 1

Actually both t and "some string" -> t will break at runtime with the same issue:

import scala.language.existentials
import scala.reflect.runtime.universe._

type SubMode[T] = M forSome { type M <: Mode[T] }

val t: MyType[_, _] = new MyType[String, SubMode[String]] {}
// t: MyType[_, _] = $anon$1@596afb2f

"some string" -> t
// error: type arguments [_$1,_$2] do not conform to trait MyType's type parameter bounds [T,M <: Mode[T]]

reify(t)
// error: type arguments [_$1,_$2] do not conform to trait MyType's type parameter bounds [T,M <: Mode[T]]

reify("some string" -> t)
// error: type arguments [_$1,_$2] do not conform to trait MyType's type parameter bounds [T,M <: Mode[T]]

In fact, the issue at compile-time is not specific to Tuples. For example:

List(t)
// error: type arguments [_$1,_$2] do not conform to trait MyType's type parameter bounds [T,M <: Mode[T]]

My guess is that while the definition of t alone does not trigger a compilation error, an additional "touch" on t might. It'll all work fine if you let the compiler infer the proper type:

val t2 = new MyType[String, SubMode[String]] {}
t2: MyType[String,SubMode[String]] = $anon$1@19d53ab4

"some string" -> t2
// res1: (String, MyType[String,SubMode[String]]) = (some string,$anon$1@19d53ab4)

List(t2)
// res2: List[MyType[String,SubMode[String]]] = List($anon$1@19d53ab4)