Typechecking extractors is not idempotent

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Pattern matching that involves extractors cannot be typechecked twice. The first typecheck distorts its structure, introducing UnApply node that wraps a synthetic SELECTOR_DUMMY ident that doesn't have a definition. If one performs resetAllAttrs, the resulting tree can no longer be typechecked.

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Imported From: https://issues.scala-lang.org/browse/SI-5465?orig=1
Reporter: @xeno-by
Affected Versions: 2.10.0
Blocks #5464

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@xeno-by said:
See tests introduced in scala/scala@5028181

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@retronym said:
Here's a potential solution: undo the wrapping Unapply(Apply(Select(qual, <unapply selector>), args)) in ResetAttrs.

https://github.com/retronym/scala/tree/ticket/5465

It solves Eugene Y.'s reported problem with extractors in macros, but does not yet fix run/idempotency-extractors.scala (with the toolbox compiler).

[SethTisue]

consolidating under #5464