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    Fix #15787 by squashing a coercion hole. · 7a439e7b
    Richard Eisenberg authored and Ben Gamari's avatar Ben Gamari committed
    In type-incorrect code, we can sometimes let a coercion
    hole make it through the zonker. If this coercion hole then
    ends up in the environment (e.g., in the type of a data
    constructor), then it causes trouble later.
    
    This patch avoids trouble by substituting the coercion hole
    for its representative CoVar. Really, any coercion would do,
    but the CoVar was very handy.
    
    test case: polykinds/T15787
    
    (cherry picked from commit 4427315a)
    7a439e7b