Regression in `frawa/typed-json` for typer / implicit search

[WojciechMazur]

Based on OpenCB failure in frawa/typed-json - https://github.com/VirtusLab/community-build3/actions/runs/10043516433/job/27756747215

Not exactly sure if it should be fixed. The compiler is now more precise which might be an improvement. I'm not sure how to workaround it though without explicitly hinting the type

Compiler version

Since 3.4.1
Last good release: 3.4.1-RC1-bin-20240126-1716bcd-NIGHTLY
First bad release: 3.4.1-RC1-bin-20240129-b20747d-NIGHTLY

Probable cause: #19096

Minimized code

sealed trait Keyword
sealed trait AssertionKeyword extends Keyword
sealed trait TypeKeyword extends AssertionKeyword
case object NullTypeKeyword extends TypeKeyword

case class Keywords(keywords: Set[Keywords.KeywordWithLocation])
object Keywords:
  type KeywordWithLocation = WithLocation[Keyword]

case class WithLocation[+T](uri: String, value: T)

trait Compare[A, B]
object Compare extends ComparePriority2:
  given compareSubtypeWithSupertype[A, B](using A <:< B): Compare[A, B] = ???
trait ComparePriority2:
  given compareSupertypeWithSubtype[A, B](using A <:< B): Compare[B, A] = ???

class KeywordsTest:
  private def assertKeywords(f: Keywords => Unit): Either[Nothing, Unit] = ???
  def assertEquals[A, B](obtained: A, expected: B)(using Compare[A, B]) = ???
  val keywords: Keywords = ???
  assertEquals(keywords.keywords, Set(WithLocation("", NullTypeKeyword))) // error
  // Summon resolves to: 
  // - in 3.4.1 Compare.compareSubtypeWithSupertype[Set[Keywords.KeywordWithLocation], Set[WithLocation[Keyword]]]
  // - in 3.4.2 Compare.compareSubtypeWithSupertype[Set[Keywords.KeywordWithLocation], Set[WithLocation[NullTypeKeyword.type]]]

Output

-- [E172] Type Error: /Users/wmazur/projects/sandbox/src/main/scala/test.scala:22:73 
22 |  assertEquals(keywords.keywords, Set(WithLocation("", NullTypeKeyword))) // error
   |                                                                         ^
   |No given instance of type Compare[Set[Keywords.KeywordWithLocation],
   |  Set[WithLocation[NullTypeKeyword.type]]] was found for parameter x$3 of method assertEquals in class KeywordsTest.
   |I found:
   |
   |    Compare.compareSubtypeWithSupertype[Set[Keywords.KeywordWithLocation],
   |      Set[WithLocation[NullTypeKeyword.type]]](
   |      /* missing */
   |        summon[Set[Keywords.KeywordWithLocation] <:<
   |          Set[WithLocation[NullTypeKeyword.type]]]
   |    )
   |
   |But no implicit values were found that match type Set[Keywords.KeywordWithLocation] <:< Set[WithLocation[NullTypeKeyword.type]].

Expectation

Not sure, but maybe it should still compile.

[mbovel]

I saw a similar problem in scalameta/munit#819.

This sounds expected if we instantiate type parameters before implicit search. So I guess the solution is to add type ascriptions manually.

[mbovel]

Further minimized:

// eq.scala
class C
class D extends C

class Compare[A, B]
given [A, B](using A <:< B): Compare[A, B] = Compare()

def assertEquals[A, B](obtained: A, expected: B)(using Compare[A, B]) = ???

@main def main = assertEquals(Set(C()), Set(D()))

➜  ~/scala-snippets-6 scala compile -S 3.4.1-RC1-bin-20240126-1716bcd-NIGHTLY eq.scala
Downloading Scala 3.4.1-RC1-bin-20240126-1716bcd-NIGHTLY compiler
Downloading Scala 3.4.1-RC1-bin-20240126-1716bcd-NIGHTLY bridge
Compiling project (Scala 3.4.1-RC1-bin-20240126-1716bcd-NIGHTLY, JVM (17))
Compiled project (Scala 3.4.1-RC1-bin-20240126-1716bcd-NIGHTLY, JVM (17))
➜  ~/scala-snippets-6 scala compile -S 3.4.1-RC1-bin-20240129-b20747d-NIGHTLY eq.scala
Downloading Scala 3.4.1-RC1-bin-20240129-b20747d-NIGHTLY compiler
Downloading Scala 3.4.1-RC1-bin-20240129-b20747d-NIGHTLY bridge
Compiling project (Scala 3.4.1-RC1-bin-20240129-b20747d-NIGHTLY, JVM (17))
[error] ./eq.scala:9:50
[error] No given instance of type Compare[Set[C], Set[D]] was found for parameter x$3 of method assertEquals.
[error] I found:
[error] 
[error]     given_Compare_A_B[Set[C], Set[D]](/* missing */summon[Set[C] <:< Set[D]])
[error] 
[error] But no implicit values were found that match type Set[C] <:< Set[D].
[error] @main def main = assertEquals(Set(C()), Set(D()))
[error]                                                  ^
Error compiling project (Scala 3.4.1-RC1-bin-20240129-b20747d-NIGHTLY, JVM (17))
Compilation failed

[mbovel]

This sounds expected if we instantiate type parameters before implicit search. So I guess the solution is to add type ascriptions manually.

We discussed it during today's compiler meeting. I confirm the new behavior is expected.

One possible workaround is to add a type ascription to the second set (or to its element):

class C
class D extends C

class Compare[A, B]
given [A, B](using A <:< B): Compare[A, B] = Compare()

def assertEquals[A, B](obtained: A, expected: B)(using Compare[A, B]) = ???

@main def main = assertEquals(Set(C()), Set(D()): Set[C])
@main def main2 = assertEquals(Set(C()), Set(D(): C))

Another workaround is to define a specific Compare instances for Sets :

class C
class D extends C

class Compare[A, B]
object Compare extends ComparePriority2:
  given compare_sup[A, B](using A <:< B): Compare[A, B] = Compare()
  given compare_sets[A, B](using Compare[A, B]): Compare[Set[A], Set[B]] = Compare()
trait ComparePriority2:
  given compare_sub[A, B](using B <:< A): Compare[A, B] = Compare()

def assertEquals[A, B](obtained: A, expected: B)(using Compare[A, B]) = ???

@main def main = assertEquals(Set(C()), Set(D()))

[WojciechMazur]

@frawa This might require changes in your codebase in the future when upgrading Scala 3 version