GDP = C + I + G + (X-M)
If GDP is supposed to count only final goods and services, why include I? Eg if build public roads, can count as G; if build factory, can count when factory produces goods and those goods are sold.
Put differently, counting I seems to double count. Eg if one counts spending on factory as I today, when the factory produces and sells goods tomorrow and in subsequent periods, the counting is done again (eg I, followed by C).