Commits on Dec 22, 2022

1. subtypes: fast path for Union/Union subtype check …

   Enums are exploded into Union of Literal when narrowed.
   
   Conditional branches on enum values can result in multiple distinct narrowing
   of the same enum which are later subject to subtype checks (most notably via
   `is_same_type`, when exiting frame context in the binder). Such checks would
   have quadratic complexity: `O(N*M)` where `N` and `M` are the number of entries
   in each narrowed enum variable, and led to drastic slowdown if any of the enums
   involved has a large number of valuees.
   
   Implemement a linear-time fast path where literals are quickly filtered, with
   a fallback to the slow path for more complex values.
   
   In our codebase there is one method with a chain of a dozen if statements
   operating on instances of an enum with a hundreds of values. Prior to the
   regression it was typechecked in less than 1s. After the regression it takes
   over 13min to typecheck. This patch fully fixes the regression for us.
   
   Fixes python#13821

   

   huguesb committed Dec 22, 2022
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