Hack: callee expression for when with subject variable is the subject
variable declaration. This solves the problem that all sub-calls in the
expression are implicitly considered to have a single common lexical
scope (and 'when (val x = ...)' introduces a new lexical scope, which
contains 'x').
'Subject.Error' is redundant.
'Subject.None' can be an object.
'Subject#dataFlowValue' can be a lateinit property.
TODO: fix
- parsing local extension properties in 'when' subject
- parsing destructuring declarations in 'when' subject
- non-completed calls in nested 'when' with subject variable
- non-completed calls for subject variable in 'in' pattern
Dmitriy Novozhilov