`@targetName` on toplevel class/trait/object fails separate compilation

[griggt]

Compiler version

3.2.0-RC1-bin-20220427-b1f00a7-NIGHTLY and earlier

Minimized code

Foo.scala

@annotation.targetName("Bar") class Foo:
  def it: Int = 42

Test.scala

@main def Test = println(Foo().it)

Output

$ scalac Foo.scala
$ scalac Test.scala
-- [E006] Not Found Error: Test.scala:1:25 -------------------------------------
1 |@main def Test = println(Foo().it)
  |                         ^^^
  |                         Not found: Foo
  |
  | longer explanation available when compiling with `-explain`
1 error found

Expectation

I understand that supporting @targetName on class/trait/object is intentional (see #11969 (comment)), thus I would expect such usage to work with separate compilation.

Remarks

No issue with joint compilation.
No issue if Foo is an inner class.

[griggt]

I don't see an efficient way to support this, since the symbol loader determines the entered symbol name from the classfile name

https://github.com/lampepfl/dotty/blob/f29eea79bb1bae6849dd9842c131b2eb1c1d1cb8/compiler/src/dotty/tools/dotc/core/SymbolLoaders.scala#L204-L205

and to do otherwise would likely require eagerly reading the classfile?

Can we just disallow @targetName on toplevel class/trait/object?

[smarter]

Hmm, this also breaks the convention that there's a Foo.tasty matching Foo.class, so I now agree that we should just disallow this.