Check macro expansion type

[mbovel]

Macro-generated trees are type-checked internally but not with respect to the expected type, such that you can write the following and get a runtime exception:

// tests/neg-macros/expr-asInstanceOf/Macro_1.scala
import scala.quoted.*

object Macro:
  inline def foo(): Int =
    ${fooImpl()}

  def fooImpl()(using Quotes): Expr[Int] =
    import quotes.reflect.*
    Expr("hello").asInstanceOf[Expr[Int]]

// tests/neg-macros/expr-asInstanceOf/Test_2.scala
@main def test =
  println(Macro.foo()) // runtime error

This PR attempts to fix that by checking the generated-tree type with respect to the parent splice type.

[mbovel]

Error in Monocle:

[error] -- Error: /home/runner/work/scala3/scala3/community-build/community-projects/Monocle/core/shared/src/test/scala-3/monocle/internal/IsoFieldsTest.scala:10:41 
[error] 10 |    val iso: Iso[Foo.type, EmptyTuple] = Iso.fields[Foo.type]
[error]    |                                         ^^^^^^^^^^^^^^^^^^^^
[error]    |Macro expansion has type monocle.Iso[Foo.type, EmptyTuple.type], which does not conform to the expected type monocle.PIso[Foo.type, Foo.type, Tuple, Tuple]
[error]    |----------------------------------------------------------------------------
[error]    |Inline stack trace
[error]    |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
[error]    |This location contains code that was inlined from IsoFields.scala:9
[error]  9 |    ${ IsoFieldsImpl.apply[S]('mirror) }
[error]    |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[error]    |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
[error]    |This location contains code that was inlined from IsoFields.scala:9
[error] 17 |      IsoFields[S]
[error]    |      ^^^^^^^^^^^^
[error]     ----------------------------------------------------------------------------
[error] one error found
[error] (coreJVM / Test / compileIncremental) Compilation failed
[error] Total time: 29 s, completed Apr 10, 2026, 4:11:55 PM

[mbovel]

Right: type Iso[S, A] = PIso[S, S, A, A], but PIso is non-variant, so monocle.PIso[Foo.type, Foo.type, EmptyTuple.type, EmptyTuple.type] is not related to monocle.PIso[Foo.type, Foo.type, Tuple, Tuple].

See also test, def fields, object IsoField.

[mbovel]

The culprit is whitebox: https://github.com/optics-dev/Monocle/blob/d49981e4623f2100d2671b6ff47bc2aaa29e7af4/core/shared/src/main/scala-3/monocle/internal/IsoFields.scala#L16-L25. This is exactly the pattern we want to prevent, so the error is legitimate.

[mbovel]

Pragmatically, maybe we should first introduce this check only when-Xcheck-macros is passed, until all community-build libraries are fixed. I changed the code to do that.

[mbovel]

Ah, we do pass -Xcheck-macros to community build projects by default:

scala3/community-build/src/scala/dotty/communitybuild/projects.scala

Lines 122 to 127 in 2b47054

	object SbtCommunityProject:
	def scalacOptions = List(
	"-Xcheck-macros",
	"-Wsafe-init",
	)

[mbovel]

Monocle PR: optics-dev/Monocle#1574.

[mbovel]

@bishabosha the CI is now green. Monocle was the only error; I updated it upstream and in the community build. Do you still approve? 😄

[bishabosha]

looks goood