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Previously we had a very aggressive Core Lint check which caught unsaturated applications of runRW#. However, there is nothing wrong with such applications and they may naturally arise in desugared Core. For instance, the desugared Core of Data.Primitive.Array.runArray# from the `primitive` package contains: case ($) (runRW# @_ @_) (\s -> ...) of ... In this case it's almost certain that ($) will be inlined, turning the application into a saturated application. However, even if this weren't the case there isn't a problem: CorePrep (after deleting an unnecessary case) can simply generate code in its usual way, resulting in a call to the Haskell definition of runRW#. Fixes #18291.
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