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.
The main problem of the previous approach was that we were only generating
erased method signatures, which was incorrect in case a class also had a member
from another supertype with the same signature as the substituted one from the
collection. Javac issues compilation errors when compiling Java code against
such classes.
Also all the needed method stub signatures were hardcoded in
generateBuiltInMethodStubs() and the case of MutableListIterator was missing
Svyatoslav Kuzmich