Allow override protected[C] in companion

[som-snytt]

When extending C in its companion, allow
override protected[C] where C denotes the
enclosing companion module of C.

Forward ports scala/scala#9814

[som-snytt]

Getting the local companion fails under -from-tasty; therefore, that half of the test is commented out. Since that is consistent with the implementation restriction in Scala 2, consider the local companion check unsupported.

[smarter]

What's going on with -from-tasty exactly here? If something can be compiled it should also work from tasty.

[som-snytt]

asking for companionModule fails for local class. Perhaps you mean, conversely, if it fails from tasty, it must also fail always. Probably there is a convenient is(Local) flag check to add to the condition.

[smarter]

asking for companionModule fails for local class

Hmm that might be a bug since I could see that leading to different behavior in other situations. Also I tried uncommenting this code and running it with testCompilation but the pickling tests didn't fail.

[som-snytt]

I'll try again, against the moving target of HEAD.

[som-snytt]

Maybe something has progressed.

Your branch and 'upstream/master' have diverged,
and have 1 and 234 different commits each, respectively.

I had to consult the page on how to test your changes:

sbt:scala3> scala3-compiler-bootstrapped/testCompilation --from-tasty t12494
[info] Test run started
[info] Test dotty.tools.dotc.FromTastyTests.runTestFromTasty started
No files matched "t12494" in test
No files matched "t12494" in test
[info] Test dotty.tools.dotc.FromTastyTests.posTestFromTasty started
[=======================================] completed (1/1, 0 failed, 1s)
-- [E164] Declaration Error: tests/pos/t12494.scala:23:25 --------------------------------------------------------------
23 |        protected[C] def f: Int = 42  // ok
   |                         ^
   |                         error overriding method f in trait C of type => Int;
   |                           method f of type => Int has weaker access privileges; it should be at least protected
Compilation failed for: 'tests/pos/t12494.scala'
[=======================================] completed (1/1, 1 failed, 0s)

Same after rebase.

[som-snytt]

Did not research the problem with getting local companion under -from-tasty, but enabled the neg test and enforce nonlocal class only, so that will fail under any compilation regime.

[smarter]

Otherwise LGTM.