Net business loss carry-forward.

Except as provided in par. (b) and s. 71.80 (25), a corporation, except a tax-option corporation or an insurer to which s. 71.45 (4) applies, may offset against its Wisconsin net business income any Wisconsin net business loss incurred in any of the 20 immediately preceding taxable years, if the corporation was subject to taxation under this chapter in the taxable year in which the loss was incurred, to the extent not offset by other items of Wisconsin income in the loss year and by Wisconsin net business income of any year between the loss year and the taxable year for which an offset is claimed. For purposes of this subsection, Wisconsin net business income or loss shall consist of all the income attributable to the operation of a trade or business in this state, less the business expenses allowed as deductions in computing net income. The Wisconsin net business income or loss of corporations engaged in business within and without the state shall be determined under s. 71.25 (6) and (10) to (12). Nonapportionable losses having a Wisconsin situs under s. 71.25 (5) (b) shall be included in Wisconsin net business loss; and nonapportionable income having a Wisconsin situs under s. 71.25 (5) (b), whether taxable or exempt, shall be included in other items of Wisconsin income and Wisconsin net business income for purposes of this subsection.