Green coffee contains sucrose, glucose and fructose; the roasted bean and the cup contain almost none. Black coffee with no sugar added has no meaningful sugar, yet it can taste distinctly sweet.
Where the sugar goes: roasting drives the Maillard reaction and caramelisation, which consume sugars and convert them into hundreds of aromatic compounds. What is left is a trace of sugar-derived molecules — and with bitterness and acidity reduced around them, the brain reads those as sweetness.
Same effect as bread: bread needs no added sugar to develop aromas that taste toasted, caramelised or sweet; the crust produces them.
The distinction worth keeping: a naturally sweet coffee is not a coffee with sugar in it. When a bag describes caramel, chocolate, honey or fruit, none of those ingredients are present — they are what the bean and the roast produced. That vocabulary describes aroma, not a sweetener.
Which is why sweetness is a quality signal: it comes from a ripe bean, a controlled roast, an extraction that stops before bitterness arrives. Off-flavours and harsh bitterness mask it.