Strangely omitted compileTimeOnly message for Unit companion

[som-snytt]

Compiler version

3.2.2

Minimized code

scala> class C { @annotation.compileTimeOnly("non-clonal") override def clone() = null }
// defined class C

scala> C().clone()
-- Error: --------------------------------------------------------------------------------------------------------------
1 |C().clone()
  |^^^^^^^^^
  |non-clonal
1 error found

scala> val u = Unit
-- Error: --------------------------------------------------------------------------------------------------------------
1 |val u = Unit
  |        ^^^^
  |        Reference to object Unit in package scala should not have survived,
  |        it should have been processed and eliminated during expansion of an enclosing macro or term erasure.
1 error found

scala>
➜  ~ scala
Welcome to Scala 2.13.10 (OpenJDK 64-Bit Server VM, Java 20.0.1).
Type in expressions for evaluation. Or try :help.

scala> val u = Unit
               ^
       error: `Unit` companion object is not allowed in source; instead, use `()` for the unit value

scala>

Expectation

Lowered.

[som-snytt]

It doesn't recognize annot.args(0) as a constant.

Some(TypeApply(Select(Literal(Constant(`Unit` companion object is not allowed in source; instead, use `()` for the unit value)),asInstanceOf),List(TypeTree[TypeRef(TermRef(ThisType(TypeRef(NoPrefix,module class scala)),object Predef),type String)])))

Perhaps it's because only some annotations were converted to ConstantAnnotation at scala/scala#9336.

That was due to auxiliary constructors, as commented on deprecatedNames.

So for example

Welcome to Scala 3.3.1-RC1-bin-SNAPSHOT-git-22b2259 (20.0.1, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.

scala> "abc".forall(f = (_: Char) => true)
-- Error: --------------------------------------------------------------------------------------------------------------
1 |"abc".forall(f = (_: Char) => true)
  |             ^^^^^^^^^^^^^^^^^^^^^
  |             method forall in class StringOps: (p: Char => Boolean): Boolean does not have a parameter f
1 error found

instead of

scala> "abc".forall(f = (_: Char) => true)
                      ^
       warning: the parameter name f is deprecated (since 2.13.3): use p instead
val res1: Boolean = true

for @deprecatedName.

The famous last words at scala/scala#9463 (comment)

Honestly no idea, I never got what ConstantAnnotation was for and Dotty doesn't treat it specially.

Not immediately obvious where the cast comes from, but maybe Scala 3 can drill past asInstanceOf in its search for an arg.

Also erasure should use annot.argumentConstantString and not convert toString.