docs/_docs/reference/error-codes/E057.md
This error is emitted when a type argument does not conform to the declared type bounds of a type parameter.
Type parameters can have upper bounds (<:) and lower bounds (>:). When you provide a type argument, it must satisfy these bounds. An upper bound means the type argument must be a subtype of the bound, while a lower bound means it must be a supertype.
trait A
trait B extends A
trait C extends B
class Contra[-T >: B]
def example =
val a: Contra[A] = ???
val b: Contra[B] = ???
val c: Contra[C] = ???
-- [E057] Type Mismatch Error: example.scala:9:18 ------------------------------
9 | val c: Contra[C] = ???
| ^
| Type argument C does not conform to lower bound B
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| I tried to show that
| B
| conforms to
| C
| but none of the attempts shown below succeeded:
|
| ==> B <: C = false
|
| The tests were made under the empty constraint
-----------------------------------------------------------------------------
// Use a types that conforms to the bound
trait A
trait B extends A
trait C extends A
class Contra[-T >: B]
def example =
val a: Contra[A] = ???
val b: Contra[B] = ???
// Or change the type bound if appropriate
trait A
trait B extends A
trait C extends B
class Contra[-T >: C]
def example =
val a: Contra[A] = ???
val b: Contra[B] = ???
val c: Contra[C] = ???