Localized failure's canonical fallback classified a failing type as an exact mapping whenever its canonical form mapped to something other than the checked dynamic. A record or array holding a union that mapCanonical collapses to the checked dynamic (an arm such as {} or unknown) then counted as exact, so its slot got no SC6104 warning and its dynamic operations no attribution.
mapCanonical now notes such a collapse (CanonMapNotes), and canonicalType refuses the form when it happened, so the type degrades through the pushed-down form like any other.
Chris Tate
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