An export subsidy is WTO-illegal.
An import tariff is WTO-illegal. A deliberately undervalued currency, maintained by massive foreign exchange intervention over a period of years, is in effect a combination of an export subsidy and an import tariff. Two wrongs, combined into a single policy, don't make a right.
Source:
http://krugman.blogs.nytimes.com/2010/06/12/the-rmb-and-the-wto/
