Cannot compile polymorphic methods with default null value under explicit nulls

[HarrisL2]

Compiler version

3.7.3-RC2

Minimized code

package issue

def foo[A](s: A = null) = ???

scalac local/implicit.scala -d local/out -classpath "local/out"

import issue.* 

val x = foo[String]()

scalac local/explicit.scala -Yexplicit-nulls -d local/out -classpath "local/out"

Output

-- [E007] Type Mismatch Error: local/explicit.scala:3:8 ------------------------
3 |val x = foo[String]()
  |        ^^^^^^^^^^^
  |Found:    Null
  |Required: String
  |Note that implicit conversions were not tried because the result of an implicit conversion
  |must be more specific than String
  |
  | longer explanation available when compiling with `-explain`
1 error found

Expectation

The default argument should not throw an error

[som-snytt]

Works with cast, which is well specified.

//> using options -Yexplicit-nulls

//def f[A](s: A = null) = ???
def f[A](s: A = null.asInstanceOf[A]) = ???

def g = f[String]()

There may be tickets about "what is the type of a default arg"; I have read that default args may be re-engineered as a feature.

The difference here is

def f$default$1[A >: Nothing <: Any]: A = null.asInstanceOf[A]

instead of

def f$default$1[A >: Nothing <: Any]: Null = null

[sjrd]

As designed. Default values are type checked at call site. It's a debatable feature, but a feature nevertheless.