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
approved these changes
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.