The fix for 6827648 introduced a bug in Types.implementation(). The fix introduced a cache in order to store method implementations (this boosted javac implementation, as javac non longer had to scan the supertype hierarchy looking for suitable implementations of methods).
Unfortunately, as the scopes are further touched by javac code (e.g. in stages like Lower or TransTypes), the cache ends up containing stale values (e.g. because meanwhile, an additional implementation could have been added to a scope). While this problem has not been noticed, its impact is significant and it is actually holding back the fix for 6337171.