Compiler crash in 3.7 nightly while compiling http4s-related code, "invalid new prefix"

[mrdziuban]

Compiler version

3.7.0-RC1-bin-20250306-73ba485-NIGHTLY

Minimized code

//> using dep org.http4s::http4s-core:0.23.30

import cats.effect.IO
import org.http4s.EntityBody

type Response = org.http4s.Response[IO]

extension (res: Response) {
  def withEntity(f: EntityBody[IO] => EntityBody[IO]): Response = res.copy(body = f(res.body))
}

Output

  unhandled exception while running MegaPhase{crossVersionChecks, firstTransform, checkReentrant, elimPackagePrefixes, cookComments, checkLoopingImplicits, betaReduce, inlineVals, expandSAMs, elimRepeated, refchecks, dropForMap} on /Users/matt/scala3.7-http4s-crash/src/main/scala/example/Test.scala

  An unhandled exception was thrown in the compiler.
  Please file a crash report here:
  https://github.com/scala/scala3/issues/new/choose
  For non-enriched exceptions, compile with -Xno-enrich-error-messages.


     while compiling: /Users/matt/scala3.7-http4s-crash/src/main/scala/example/Test.scala
        during phase: MegaPhase{crossVersionChecks, firstTransform, checkReentrant, elimPackagePrefixes, cookComments, checkLoopingImplicits, betaReduce, inlineVals, expandSAMs, elimRepeated, refchecks, dropForMap}
                mode: Mode(ImplicitsEnabled)
     library version: version 2.13.16
    compiler version: version 3.7.0-RC1-bin-20250306-73ba485-NIGHTLY-git-73ba485
            settings: -classpath /Users/matt/scala3.7-http4s-crash/target/scala-3.7.0-RC1-bin-20250306-73ba485-NIGHTLY/classes:/Users/matt/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/scala-lang/scala3-library_3/3.7.0-RC1-bin-20250306-73ba485-NIGHTLY/scala3-library_3-3.7.0-RC1-bin-20250306-73ba485-NIGHTLY.jar:/Users/matt/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/http4s/http4s-core_3/0.23.30/http4s-core_3-0.23.30.jar:/Users/matt/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/scala-lang/scala-library/2.13.16/scala-library-2.13.16.jar:/Users/matt/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/typelevel/case-insensitive_3/1.4.2/case-insensitive_3-1.4.2.jar:/Users/matt/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/typelevel/cats-core_3/2.11.0/cats-core_3-2.11.0.jar:/Users/matt/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/typelevel/cats-effect-std_3/3.5.7/cats-effect-std_3-3.5.7.jar:/Users/matt/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/typelevel/cats-parse_3/1.0.0/cats-parse_3-1.0.0.jar:/Users/matt/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/http4s/http4s-crypto_3/0.2.4/http4s-crypto_3-0.2.4.jar:/Users/matt/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/co/fs2/fs2-core_3/3.11.0/fs2-core_3-3.11.0.jar:/Users/matt/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/co/fs2/fs2-io_3/3.11.0/fs2-io_3-3.11.0.jar:/Users/matt/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/com/comcast/ip4s-core_3/3.6.0/ip4s-core_3-3.6.0.jar:/Users/matt/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/typelevel/literally_3/1.1.0/literally_3-1.1.0.jar:/Users/matt/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/scodec/scodec-bits_3/1.1.38/scodec-bits_3-1.1.38.jar:/Users/matt/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/typelevel/vault_3/3.6.0/vault_3-3.6.0.jar:/Users/matt/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/log4s/log4s_3/1.10.0/log4s_3-1.10.0.jar:/Users/matt/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/typelevel/log4cats-slf4j_3/2.7.0/log4cats-slf4j_3-2.7.0.jar:/Users/matt/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/typelevel/cats-kernel_3/2.11.0/cats-kernel_3-2.11.0.jar:/Users/matt/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/typelevel/cats-effect-kernel_3/3.5.7/cats-effect-kernel_3-3.5.7.jar:/Users/matt/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/typelevel/cats-effect_3/3.5.4/cats-effect_3-3.5.4.jar:/Users/matt/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/slf4j/slf4j-api/1.7.36/slf4j-api-1.7.36.jar:/Users/matt/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/typelevel/log4cats-core_3/2.7.0/log4cats-core_3-2.7.0.jar -d /Users/matt/scala3.7-http4s-crash/target/scala-3.7.0-RC1-bin-20250306-73ba485-NIGHTLY/classes

## Exception when compiling 1 sources to /Users/matt/scala3.7-http4s-crash/target/scala-3.7.0-RC1-bin-20250306-73ba485-NIGHTLY/classes
dotty.tools.dotc.core.TypeError$$anon$1: invalid new prefix  = org.http4s.Response[cats.effect.IO] cannot replace (Message.this : org.http4s.Message[F] & org.http4s.Message[F]) in type Message.this.Self

Expectation

It should compile without any error, as in previous versions

[mrdziuban]

Interestingly, without the type Response alias there's no error, i.e. this compiles fine

import cats.effect.IO
import org.http4s.{EntityBody, Response}

extension (res: Response[IO]) {
  def withEntity(f: EntityBody[IO] => EntityBody[IO]): Response[IO] = res.copy(body = f(res.body))
}

[mrdziuban]

Possibly related to #22727 since that also uses a type alias and an extension method, cc @hamzaremmal

[Gedochao]

Last good release: 3.7.0-RC1-bin-20250222-4dc4668-NIGHTLY
First bad release: 3.7.0-RC1-bin-20250225-a25fe5e-NIGHTLY
The first bad commit is 7d79c56
cc @hamzaremmal

Possibly related to #22727 since that also uses a type alias and an extension method, cc @hamzaremmal

Suspecting it might be a duplicate, we'd need to minimize it without the dependency to be sure.
Leaving it open until we confirm.

[som-snytt]

I'm checking the extension check right now, as I have an open PR that touches that code. I'll update in a moment.

[som-snytt]

I confirm it fails with 3.nightly 3.7.0-RC1-bin-20250306-73ba485-NIGHTLY-git-73ba485

but not with local #22708

I'll tweak to convince myself that the PR is the fix.

[som-snytt]

OK, I confirmed that it crashes without the tweak in the PR.

This is not a duplicate ticket (for that PR), so I'll reference this ticket. I see you mean it duplicates another report!