Ideally, the type of `IrWhen` should be provided by type inference for
a consistent behavior. `USED_AS_EXPRESSION` from CFG isn't always
consistent with type inference, unfortunately.
The behavior is now aligned with `if`. The type of `when` is kept when
it *can* be an expression, instead of whether it is used or not.
* In blocks, discard the result of any statement that has a return
type other than void. This was previously done by wrapping each
statement into an "implicit Unit conversion" that was actually
compiled down to a stack pop instead. If an expression happened to
already have type Unit, however, such a conversion was not inserted,
resulting in a stray reference on the stack. These conversions are
now redundant and should probably be removed.
* In assignments and non-exhaustive conditionals, materialize a Unit
on the stack to avoid depth mismatches that trip up the bytecode
validator. Because such expressions are generally used at block level
(and, indeed, the frontend will reject a non-exhaustive conditional
used as an expression), combined with the above change this results
in no additional GETSTATIC opcodes, as they are immediately removed
by the peephole optimizer.
Ting-Yuan Huang