Stuck on an issue?

Lightrun Answers was designed to reduce the constant googling that comes with debugging 3rd party libraries. It collects links to all the places you might be looking at while hunting down a tough bug.

And, if you’re still stuck at the end, we’re happy to hop on a call to see how we can help out.

BitSet does not conform to Set[Int] in match type

See original GitHub issue

Compiler version

3.1.3-RC1-bin-20220228-fa08d00-NIGHTLY

Minimized code

import scala.collection.immutable.BitSet

type SetFor[A] <: Set[A] = A match
  case Int => BitSet
  case A => Set[A]

Output

  case Int => BitSet
              ^^^^^^
              Found:    collection.BitSet
              Required: Set[A]

Expectation

Compile and match correctly.

Issue Analytics

State:
Created 2 years ago
Comments:13 (8 by maintainers)

Top GitHub Comments

1reaction

dwijnandcommented, Mar 2, 2022

Btw, I also tried to find something that works and failed, and can’t tell exactly why it fails while LeafElem and leafElem work. Perhaps it’s some solace. 🙂 I’m not never sure where match type questions are being asked and answered…

1reaction

dwijnandcommented, Mar 1, 2022

I assume there was just an error in reproduction or minimisation and you meant scala.collection.immutable.BitSet, as in:

import scala.collection.immutable.BitSet

type SetFor[A] <: Set[A] = A match
  case Int => BitSet
  case A => Set[A]

Here’s the -explain output:

-- [E007] Type Mismatch Error: tests/pos/i14593.scala:2:41 -----------------------------------------------------------------------
2 |  case Int => scala.collection.immutable.BitSet
  |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |              Found:    collection.immutable.BitSet
  |              Required: Set[A]
  |-------------------------------------------------------------------------------------------------------------------------------
  | Explanation (enabled by `-explain`)
  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
  |
  | Tree: scala.collection.immutable.BitSet
  | I tried to show that
  |   collection.immutable.BitSet
  | conforms to
  |   Set[A]
  | but the comparison trace ended with `false`:
  |
  |   ==> collection.immutable.BitSet  <:  Set[A]
  |     ==> Set[Int]  <:  Set[A] (left is approximated)
  |       ==> (scala.collection.immutable : scala.collection.immutable.type)  <:  collection.immutable.type
  |       <== (scala.collection.immutable : scala.collection.immutable.type)  <:  collection.immutable.type = true
  |       ==> A  <:  Int
  |         ==> Any  <:  Int (left is approximated)
  |         <== Any  <:  Int (left is approximated) = false
  |       <== A  <:  Int = false
  |     <== Set[Int]  <:  Set[A] (left is approximated) = false
  |   <== collection.immutable.BitSet  <:  Set[A] = false
  |
  | The tests were made under the empty constraint
   -------------------------------------------------------------------------------------------------------------------------------

So looks like when A is matched with Int, A := Int (or perhaps A =:= Int) isn’t added to the constraints such that this is true.