compound-type-weirdness.scala

scala> trait Foo[A] { def bar[B](b: B): Foo[A with B] }
defined trait Foo

scala> def baz[A](foo: Foo[A]) = foo bar 1
baz: [A](foo: Foo[A])Foo[A with Int]

scala> def baz[A](foo: Foo[A]): Foo[A with Int] = foo bar 1
<console>:8: error: type mismatch;
 found   : Int(1)
 required: Nothing
       def baz[A](foo: Foo[A]): Foo[A with Int] = foo bar 1
                                                          ^

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