Fix #2982: Avoid instantiating to Nothing in some situations #3834
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If variance == 0, we have to choice whether to instantiate a type variable to its lower or upper bound. We used to always pick the lower bound, but now we pick the upper bound if the lower bound is Nothing.
smarter
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Jan 14, 2018
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Note that this kind of duplicate the existing logic in IsFullyDefinedAccumulator (https://github.com/dotty-staging/dotty/blob/1ab60276f5684ec7c80f3fed3c0f52ab491ecca6/compiler/src/dotty/tools/dotc/typer/Inferencing.scala#L115), I plan to try to share more core between it and interpolateUndetVars, but until then this is fine I guess.
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If variance == 0, we have to choice whether to instantiate a type variable
to its lower or upper bound. We used to always pick the lower bound, but now
we pick the upper bound if the lower bound is Nothing.