Underspecified type variables should not infer UninhabitedType #3032

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1. ilevkivskyi

   added

   bugmypy got something wrongmypy got something wrong

   priority-1-normal

   topic-type-variables

   on Apr 14, 2017
2. pkch commented on Apr 14, 2017

   

   pkch

   on Apr 14, 2017

   Last edited by pkch

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   (Sorry, heavily edited... it's more complex than I thought.)

   AFAIU, the rule for inferring the type of lvalue in an assignment is intended to be roughly this:

   1. Use the knowledge of lvalue type to obtain constraints on all the type variables in rvalue type.
   2. Within those constraints, find concrete types for the type variables that result in the lowest possible final type. Replace type variables with those concrete types if not <uninhabited>.
   3. If a concrete type found for a type variable is <uninhabited>, but the constraint is not equality, leave the type variable unsubstituted, so it becomes a type parameter in the final result
   4. If any <uninhabited> remains, mark the code as unreachable.

   I think the problem is that mypy sometimes ends up not doing step 3, instead leaving <uninhabited> in the inferred type. This leads to confusing messages and incorrect tagging of code as unreachable.

   That said, I don't completely understand step 2, since the "lowest" type may not exist; but I hope the rest of my comments are still correct.