too eager widening of singleton types

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I expected the following code to compile:

trait Foo {
  type Arg
  type Prod
  def makeProd(a : Arg): Prod
}

// simulate dependent method types
case class buildProd(foo: Foo) {
  def apply(a: foo.Arg): foo.Prod = foo.makeProd(a)
}

object main {
  val myFoo = new Foo{type Arg=Int; type Prod=(Int, Int); def makeProd(i: Int) = (i, i)}
  
  buildProd(myFoo)(1)
}

Instead, the compiler complains:

15: error: method apply cannot be accessed in buildProd
 because its instance type (Foo#Arg)Foo#Prod contains a malformed type: Foo#Arg
  buildProd(myFoo)(1)
           ^
one error found

It seems foo.type gets approximated to Foo too early..

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Imported From: https://issues.scala-lang.org/browse/SI-102?orig=1
Reporter: @adriaanm

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@odersky said:
This would require dependent method types. They are not currently in Scala. I'll keep the ticket open as a request for enhancement.

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@adriaanm said:
I fiddled with this some more, and adding a type parameter TFoo to buildProd is enough to get the program to type check. This makes me think the original problem is not directly related to dependent method types (in fact, it encodes them, but I don't think it uses them).

case class buildProd[TFoo <: Foo](foo: TFoo) {  // now it works
  def apply(a: foo.Arg): foo.Prod = foo.makeProd(a)
}

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@odersky said:
But don't you need to parameterize the call also, as in:

   buildProd[myFoo.tpe](myFoo)

?

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@adriaanm said:
Replying to [comment:3 odersky]:

But don't you need to parameterize the call also, as in:

    buildProd[myFoo.tpe](myFoo)

?
In a more complicated example, yes -- here, the following is inferred (-Xprint:typer):

    buildProd[java.lang.Object with Foo{type Arg = Int; type Prod = (Int, Int)}](main.this.myFoo).apply(1)

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@odersky said:
Milestone postponed deleted

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@soc said:
In 2.10.0.r24516-b20110320020044 it has changed to just a type mismatch:

scala>   buildProd(myFoo)(1)
<console>:12: error: type mismatch;
 found   : Int(1)
 required: _4.foo.Arg where val _4: buildProd
         buildProd(myFoo)(1)
                          ^

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@paulp said:
I'm adding a test and closing this; if someone thinks there is an interesting space between what the test accomplishes and what we should be accomplishing, go ahead and reopen it.

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Commit Message Bot (anonymous) said:
(extempore in r25907) Test case closes #102.

No review.