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466b7d29f avoided quadratic complexity in Erasure's treatment
of chained `asInstanceOf` calls. It did so by using the typechecked
qualifier, rather than discarding it.
However, that also dropped the cast altogether! In many cases this
was masked by later inclusion of a cast to the expected type
by `adaptToType`:
at scala.tools.nsc.transform.Erasure$Eraser.cast(Erasure.scala:636)
at scala.tools.nsc.transform.Erasure$Eraser.scala$tools$nsc$transform$Erasure$Eraser$$adaptToType(Erasure.scala:665)
at scala.tools.nsc.transform.Erasure$Eraser.adapt(Erasure.scala:766)
at scala.tools.nsc.typechecker.Typers$Typer.runTyper$1(Typers.scala:5352)
This commit re-wraps the typechecked `qual` in its original
`<qual>.asInstanceOf[T]` to preserve semantics while avoiding
the big-O blowup.
The test includes the compiler option `-Ynooptimize` because dead code
elimination *also* thinks that this cast is superfluous. I'll follow up
on that problem seprately.
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