Disallow case classes defined in inline methods

[nicolasstucki]

The motivations are

• Disallow code that looks inoffensive small but has a large code size impact when inlined. We can still implement the same but must be explicit. Or implement those case classes outside of the method and avoid duplication.
• Simplify implementation of Expand non-transparent macros after Typer #9984

[liufengyun]

LGTM

[liufengyun]

Case classes are just desugarings. Maybe generalize it to all TypeDef?

[nicolasstucki]

What do you mean by generalizing? Do you mean not allowing these ones?

'{ class Bar extends Foo { def foo = ... }; ... }
'{ type T = ...;  }

Those are necessary.

[liufengyun]

If we don't generalize, the rule seems arbitrary and will not do the job. If a programmer just hand-crafted the desugaring of a case class, doesn't it have the same effect as a case class?