Quick quiz:
Q: In a CFG, what does `a -> b -> c -> d` mean?
A: `a`, then `b`, then `c`, then `d`.
Q: In a CFG, what does `a -> b -> d; a -> c -> d` mean?
A: `a`, then `b` or `c`, then `d`.
Q: So how do you encode "a, then (b, then c) or (c, then b), then d`?
A: You can't.
Problem is, you need to, because that's what `a; run2({ b }, { c }); d`
does when `run2` has a contract that it calls both its lambda arguments
in-place: `shuffle(listOf(block1, block2)).forEach { it() }` is a
perfectly valid implementation for it, as little sense as that makes.
So that's what union nodes solve. When a node implements
`UnionNodeMarker`, its inputs are interpreted as "all visited in some
order" instead of the normal "one of the inputs is visited".
Currently this is used for data flow. It *should* also be used for
control flow, but it isn't. But it should be. But that's not so easy.
BTW, `try` exit is NOT a union node; although lambdas in one branch can
be completed according to types' of lambdas in another, data does not
flow between the branches anyway (since we don't know how much of the
`try` executed before jumping into `catch`, and `catch`es are mutually
exclusive) so a `try` expression is more like `when` than a function
call with called-in-place-exactly-once arguments. The fact that
`exitTryExpression` used `processUnionOfArguments` in a weird way
should've hinted at that, but now we know for certain.
For example:
foo(
// `if` joins A & B
if (condition)
run { ... } // A
else
run { ... }, // B
run { ... } // C
) // `foo` unifies `A & B` and `C`, so if it is not resolved itself,
// further `if`s, `when`s, safe calls outside it, etc. continue
// building the correct type predicate until the next completed
// call.
^KT-44512 Fixed
Level of CFGNode is used to determine which call is a common one for
creating node with union of arguments (to merge flow from multiple
in-place lambdas). Before this change calls in different when branches
may have same node level, which entail passing smartcasts from moddle of
one branch to another
```
val x: Any = ...
when {
... -> run { x as String } // (1)
... -> {
run {
x.foo()
} // (2)
"hello"
}
}
```
Call `(1)` was assumed as argument of call `(2)` which is incorrect
#KT-44814 Fixed
pyos