It's my limited understanding that mashing corn will also produce glucose a simple sugar that will be 100% attenuated by the yeast.Replace this with the flake corn with dextrose or rice solids. The corn could also be replaced with table sugar, the only sugar available that could give you anything outside of alcohol and cO2 is rice solids. Both dextrose (corn sugar) and table sugar will ferment nearly completely and the yeast will produce mostly alcohol and cO2. The extra alcohol will dry the beer out and lighten it's body. Not 100% sure about rice solids, but I believe they have more complex sugars. Maybe someone who knows more about this can chime in.
Flaked corn, rice, wheat need to be mashed as others have stated. The difference between adding the sugar directly into the boil and mashing it is the sugar type that ends up in the beer. Adjuncts added to the mash will produce mostly maltose sugar. This is often misunderstood by brewers, believing that adding adjuncts will produce simpler sugars than barley malt. The mashing process will produce maltose from gelatinized starch regardless of it's source because of the amylase enzyme . Some simpler sugars are produced, but it's not nearly as fermentable as dextrose or table sugar.
Bottom line is that adding sugars directly to the boil is the closest thing you can get to mashing adjuncts, but the mashed adjuncts will not attenuate as much as dextrose and table sugar.
Hence why our distilling brethren like to make corn liquor with it.
The rice syrup I've been using ferments out 100% as well I believe that's mostly glucose as well.