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SI-8460 Fix regression in divergent implicit recovery
Implicit search detects likely cycles by looking at the stack of open implicits and checking the same implicit appears twice, and if the second occurrence is trying satisfy an implicit search for a "dominant" type. Originally, this condition immediately failed the entire implicit search. However, since Scala 2.10, this mechanism has been refined to continue searching after the first divergent implicit is detected. If a second divergence is found, we fail immediately. If the followup search fails, we report the first divergence. Otherwise, we take the successful result. This mechanism was originally built around exceptions. This proved to be fragile, and was refactored in SI-7291 / accaa31 to instead use the `Context.errors` to control the process. But, since that change, the pattern of implicits in scalanlp/breeze and Shapeless have been prone to reporting the divergent implicit errors where they used to recover. So long as we left the `DivergentImplictTypeError` that originates from a nested implicit search in `context.errors`, we are unable to successfully typecheck other candidates. This commit instead stashes the first such error away in `DivergentImplicitRecovery`, to clear the way for the alternative path to succeed. We must retain any other divergent implicit errors, as witnessed by test/files/neg/t2031.scala, which loops unless we retain divergent implicit errors that we don't stash in `DivergentImplicitRecovery`.
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