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This PR fixes three issues related to narrowing of types with the instanceof operator and user defined type guards:
x instanceof T
where x is of a non-union type, we now narrow the type ofx
tonever
in the false branch if the type of x is not a subtype of T. Previously we didn't change the type ofx
in the false branch.x instanceof T
where the types of x and T are both assignable to each other, we now pick T only if it is a subtype of the type of x. Previously we would always pick T.These fixes also affect similar scenarios with user defined type predicates (since the narrowing logic is shared).
Fixes #10145, #10167.