Implicit is not considered if method in question has a(n unused) type parameter

[scabug]

Consider this code, defining a simple class with a method named **:

class Foo() {
  def **(s: String) = "Foo"
}

Now we add a new classes and correspondent implicits to "add more supported types" to Foos ** method:

class Weird(foo: Foo) {
  def **[M](w: Weird) = new Weird(foo)
}

implicit def foo2weird(foo: Foo) = new Weird(foo)

class Weirder[T](foo: T) {
  def **[M](w: Weirder[T]) = new Weirder(foo)
}

implicit def foo2weirder(foo: Foo) = new Weirder(foo)

class Weirdest[T](foo: T) {
  def **(w: Weirdest[T]) = new Weirdest(foo)
}

implicit def foo2weirdest(foo: Foo) = new Weirdest(foo)

While the first and the last work, the second one fails:

// Works:
foo ** new Weird(new Foo)

// Fails:
foo ** new Weirder(new Foo)
/*
error: type mismatch;
 found   : Weirder[Foo]
 required: String
              f ** new Weirder(new Foo)
                   ^
*/

// Works again:
foo ** new Weirdest(new Foo)

The only difference between the second and the third example is the removal of the (unused) type parameter M.

It looks like as if the implicit method didn't even get considered when it had the type parameter.

This bug report is a result of http://stackoverflow.com/questions/7649517/

I already tried to read the spec to figure out if this is as expected or if this is a bug, but especially in 6.26.3 I haven't understood it fully.

I hope this can be fixed, because this behavior precludes any possibility of working with numbers where one of arguments is a parametrized type.

So while 5 * BigInt(5) will work, things like 5 * Complex[Long](...) (math), 5 * Currency[€](...) (money), 5 * QuantityMass` will not be possible.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-5107?orig=1
Reporter: @soc
Affected Versions: 2.9.2, 2.10.0

[scabug]

@soc said:
It seems that it doesn't even consider the implicit **, because scala -Xlog-implicits doesn't print anything.