The difference is how we deal with intermediate fake overrides
E.g., in case
interface A { /* $1 */ fun foo() }
interface B : A {
/* $2 */ fake_override fun foo()
}
interface C : B {
/* $3 */ override fun foo()
}
We've got FIR declarations only for $1 and $3, but we've got
a fake override for $2 in IR.
Previously, override $3 had $1 as its overridden IR symbol, just because
FIR declaration of $3 doesn't know anything about $2.
Now, when generating IR for $2, we save the necessary information
and using it for $3, so it has $2 as overridden.
So, it's consistent with the overridden structure of FE 1.0 and this
structure is necessary prerequisite for proper building of bridges
for special built-ins.
Special bridges which are generated as a replacement for a non-final
fake override and use invokespecial to call the already existing
implementation in the superclass, should not be final. Otherwise we
can't generate an override for the original Kotlin function in a
subclass with the same JVM signature.
Alexander Udalov