[SOLVED] Value assigned to type member via method type parameter breaks type equivalence

Value assigned to type member via method type parameter breaks type equivalence

Why does the following type equivalence hold

trait Foo { type T }
val fa = new Foo { type T = Int }

implicitly[fa.T =:= Int] // OK

but when type member T is assigned via method parameter A then type equivalence does not hold

def makeFoo[A]: Foo = new Foo { type T = A }
val fb = makeFoo[Int]

implicitly[fb.T =:= Int] // ERROR

Intuitively I would expect if T = A and A = Int then T = Int?

Solution

This is because the return type of makeFoo is only Foo and not Foo { type T = A }. If you don't explicitly declare it or if you refine it, the error will go away.

def makeFoo[A] = new Foo { type T = A }
val fb = makeFoo[Int]

implicitly[fb.T =:= Int] // No more error

or

def makeFoo2[A]: Foo { type T = A } = new Foo { type T = A }
val fc: Foo { type T = Int } = makeFoo[Int]

implicitly[fc.T =:= Int] // No more error

Scastie:

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