Accommodate adapted Scala 2 annotation value

[som-snytt]

Fixes #17515

Regression test for the standard library Unit, ingested in Scala 3.

When unpickling a Scala 2 annotation argument, the annotation constructor is overloaded and the argument winds up adapted, "constant".asInstanceOf[String]. Strip the cast.

[som-snytt]

looks spurious

Test 'tests\run-tasty-inspector\i14785.scala' failed with output:               
List(out\runWithCompiler\run-tasty-inspector\i14785, C:\Windows\ServiceProfiles\NetworkService\AppData\Local\Coursier\Cache\v1\https\repo1.maven.org\maven2\org\scala-lang\scala-library\2.13.16\scala-library-2.13.16.jar, C:\actions-runner2\_work\scala3\scala3\library\..\out\bootstrap\scala3-library-bootstrapped\scala-3.7.1-RC1-bin-SNAPSHOT-nonbootstrapped\scala3-library_3-3.7.1-RC1-bin-SNAPSHOT.jar, C:\Windows\ServiceProfiles\NetworkService\AppData\Local\Coursier\Cache\v1\https\repo1.maven.org\maven2\org\scala-lang\scala-library\2.13.16\scala-library-2.13.16.jar, C:\Windows\ServiceProfiles\NetworkService\AppData\Local\Coursier\Cache\v1\https\repo1.maven.org\maven2\org\scala-lang\modules\scala-asm\9.7.1-scala-1\scala-asm-9.7.1-scala-1.jar, C:\Windows\ServiceProfiles\NetworkService\AppData\Local\Coursier\Cache\v1\https\repo1.maven.org\maven2\org\jline\jline-terminal\3.29.0\jline-terminal-3.29.0.jar, C:\Windows\ServiceProfiles\NetworkService\AppData\Local\Coursier\Cache\v1\https\repo1.maven.org\maven2\org\jline\jline-reader\3.29.0\jline-reader-3.29.0.jar, C:\Windows\ServiceProfiles\NetworkService\AppData\Local\Coursier\Cache\v1\https\repo1.maven.org\maven2\org\scala-sbt\compiler-interface\1.10.7\compiler-interface-1.10.7.jar, C:\actions-runner2\_work\scala3\scala3\interfaces\target\scala3-interfaces-3.7.1-RC1-bin-SNAPSHOT.jar, C:\actions-runner2\_work\scala3\scala3\library\..\out\bootstrap\scala3-library-bootstrapped\scala-3.7.1-RC1-bin-SNAPSHOT-nonbootstrapped\scala3-library_3-3.7.1-RC1-bin-SNAPSHOT.jar, C:\actions-runner2\_work\scala3\scala3\tasty\..\out\bootstrap\tasty-core-bootstrapped\scala-3.7.1-RC1-bin-SNAPSHOT-nonbootstrapped\tasty-core_3-3.7.1-RC1-bin-SNAPSHOT.jar, C:\actions-runner2\_work\scala3\scala3\compiler\..\out\bootstrap\scala3-compiler-bootstrapped\scala-3.7.1-RC1-bin-SNAPSHOT-nonbootstrapped\scala3-compiler_3-3.7.1-RC1-bin-SNAPSHOT.jar, C:\actions-runner2\_work\scala3\scala3\tasty-inspector\..\out\bootstrap\scala3-tasty-inspector\scala-3.7.1-RC1-bin-SNAPSHOT-nonbootstrapped\scala3-tasty-inspector_3-3.7.1-RC1-bin-SNAPSHOT.jar, out\runWithCompiler\run-tasty-inspector\i8389, C:\Windows\ServiceProfiles\NetworkService\AppData\Local\Coursier\Cache\v1\https\repo1.maven.org\maven2\org\scala-lang\scala-library\2.13.16\scala-library-2.13.16.jar, C:\actions-runner2\_work\scala3\scala3\library\..\out\bootstrap\scala3-library-bootstrapped\scala-3.7.1-RC1-bin-SNAPSHOT-nonbootstrapped\scala3-library_3-3.7.1-RC1-bin-SNAPSHOT.jar, C:\Windows\ServiceProfiles\NetworkService\AppData\Local\Coursier\Cache\v1\https\repo1.maven.org\maven2\org\scala-lang\scala-library\2.13.16\scala-library-2.13.16.jar, C:\Windows\ServiceProfiles\NetworkService\AppData\Local\Coursier\Cache\v1\https\repo1.maven.org\maven2\org\scala-lang\modules\scala-asm\9.7.1-scala-1\scala-asm-9.7.1-scala-1.jar, C:\Windows\ServiceProfiles\NetworkService\AppData\Local\Coursier\Cache\v1\https\repo1.maven.org\maven2\org\jline\jline-terminal\3.29.0\jline-terminal-3.29.0.jar, C:\Windows\ServiceProfiles\NetworkService\AppData\Local\Coursier\Cache\v1\https\repo1.maven.org\maven2\org\jline\jline-reader\3.29.0\jline-reader-3.29.0.jar, C:\Windows\ServiceProfiles\NetworkService\AppData\Local\Coursier\Cache\v1\https\repo1.maven.org\maven2\org\scala-sbt\compiler-interface\1.10.7\compiler-interface-1.10.7.jar, C:\actions-runner2\_work\scala3\scala3\interfaces\target\scala3-interfaces-3.7.1-RC1-bin-SNAPSHOT.jar, C:\actions-runner2\_work\scala3\scala3\library\..\out\bootstrap\scala3-library-bootstrapped\scala-3.7.1-RC1-bin-SNAPSHOT-nonbootstrapped\scala3-library_3-3.7.1-RC1-bin-SNAPSHOT.jar, C:\actions-runner2\_work\scala3\scala3\tasty\..\out\bootstrap\tasty-core-bootstrapped\scala-3.7.1-RC1-bin-SNAPSHOT-nonbootstrapped\tasty-core_3-3.7.1-RC1-bin-SNAPSHOT.jar, C:\actions-runner2\_work\scala3\scala3\compiler\..\out\bootstrap\scala3-compiler-bootstrapped\scala-3.7.1-RC1-bin-SNAPSHOT-nonbootstrapped\scala3-compiler_3-3.7.1-RC1-bin-SNAPSHOT.jar, C:\actions-runner2\_work\scala3\scala3\tasty-inspector\..\out\bootstrap\scala3-tasty-inspector\scala-3.7.1-RC1-bin-SNAPSHOT-nonbootstrapped\scala3-tasty-inspector_3-3.7.1-RC1-bin-SNAPSHOT.jar, out\runWithCompiler\run-tasty-inspector\isSuperAccessor, C:\Windows\ServiceProfiles\NetworkService\AppData\Local\Coursier\Cache\v1\https\repo1.maven.org\maven2\org\scala-lang\scala-library\2.13.16\scala-library-2.13.16.jar, C:\actions-runner2\_work\scala3\scala3\library\..\out\bootstrap\scala3-library-bootstrapped\scala-3.7.1-RC1-bin-SNAPSHOT-nonbootstrapped\scala3-library_3-3.7.1-RC1-bin-SNAPSHOT.jar, C:

[som-snytt]

This seems to be an artifact of compilation by Scala 2, either the way Scala 2 writes the attribute or the way Scala 3 unpickles it.

[som-snytt]

I don't know yet if this is a separate issue:

4 |@implicitNotFound("nope": @nowarn)
  |                  ^^^^^^^^^^^^^^^
  |            @implicitNotFound requires constant expressions as a parameter

It compiles with 2 and is consumed correctly by 3.

[odersky]

This must hold the record for longest outstanding review ending in a simple approval. Sorry about that! And amazing there no conflicts popped up in the meantime. @som-snytt If you have nothing to add this PR, please merge.