OverridingPairsChecker

We can exclude pairs safely from checking only under three additional conditions

• their signatures also match in the parent class. See neg/i12828.scala for an example where this matters.
• They overriding/overridden appear in linearization order. See neg/i5094.scala for an example where this matters.
• They overriding/overridden appear in linearization order, or the parent is a Java class (because linearization does not apply to java classes). See neg/i5094.scala and pos/i18654.scala for examples where this matters.
• The overridden symbol is not abstract override. For such symbols we need a more extensive test since the virtual super chain depends on the precise linearization order, which might be different for the subclass. See neg/i14415.scala.

Does sym1 match sym2 so that it qualifies as overriding when both symbols are seen as members of self? Types always match. Term symbols match if their membertypes relative to self do.

If not null: The symbol overridden by overriding

The current candidate symbol for overriding