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trait MapLookup<Q> { type MapValue; } impl<K> MapLookup<K> for K { type MapValue = K; } trait Map: MapLookup<<Self as Map>::Key> { type Key; } impl<K> Map for K { type Key = K; } fn main() { let _ = &() as &dyn Map<Key=u32,MapValue=u32>; //~^ ERROR E0038 }