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Also closes SI-5158 (debuggability), SI-6070 (soundness).
To improve both debuggability and soundness,
we now store the result of an extractor (user-defined and synthetic)
in local variables.
For the case class case, this also fixes the soundness bug SI-6070,
as this prevents post-match mutation of bound variables.
The core of the refactoring consisted of introducing the PreserveSubPatBinders
trait, which introduces local variables instead of substituting symbols
for the RHS of those variables (so this can be seen as reverting the premature optimization
of inline the case-class getters into the case body).
Since TreeMakerToCond fuses the substitutions performed in a match to find out
which symbolic values binders evaluate to, masquerade PreserveSubPatBinders's
binding of subPatBinders and subPatRefs as the corresponding substitution.
Consider `case class Foo(bar: Int)`, then `case y@Foo(x) => println(x)`
gives rise to `{val x = y.bar; println(x)}` (instead of `println(y.bar)`),
and `subPatternsAsSubstitution` pretends we still replace `x` by `y.bar`,
instead of storing it in a local variable so that the rest of the analysis need not be modified.
Misc notes:
- correct type for seq-subpattern
- more error resilience (ill-typed patterns sometimes slip past the typechecker -- reopened SI-4425)
TODO: come up with a more abstract framework for expressing bound symbols and their values
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