From the Interim Review section of the Aircraft report:
6.20 According to the United States, the last sentence of paragraph 7.46 of the Interim Report may be read to suggest that the Panel is applying Article 28 of the Vienna Convention on the Law of Treaties (VCLT) as a rule of international law to adjudicate the rights and obligations of Members under the covered agreements. The United States argues that panels and the Appellate Body cannot apply rules of international law other than the provisions of the WTO Agreement. The United States submits that, pursuant to Article 3.2 of the DSU, panels do not clarify rules of international law that are not found in the covered agreements, although such rules may be relevant to clarifying the provisions of the covered agreements under the customary rules of interpretation of public international law, specifically as reflected in Article 31.3(c) of the VCLT.1474 The United States suggests that the Panel replace the last sentence in paragraph 7.46 of the Interim Report with a proposed text it considers to be a more appropriate formulation of the relationship between Article 5 of the SCM Agreement and Article 28 of the VCLT.1475
6.21 The European Communities objects to the United States' request to modify the last sentence of paragraph 7.46 of the Interim Report.1476 First, the European Communities considers that the United States' concern is fully addressed in footnote 1536 at the end of paragraph 7.46 of the Interim Report, which states that the Appellate Body has said in previous disputes that the principle codified in Article 28 of the VCLT is relevant to the interpretation of the covered agreements. Second, the European Communities argues that the Appellate Body has qualified Article 28 of the VCLT as a "general principle of international law", with the result that the United States is mistaken in arguing that Article 28 can only be taken into account in the context of Article 31.3(c) of the VCLT.1477
6.22 The principle of non-retroactivity embodied in Article 28 of the VCLT has been recognized by the Appellate Body to be a "general principle of international law" relevant to the interpretation of obligations contained in the WTO Agreements in many disputes. The United States' comment appears to have given rise to a disagreement between the parties as to the basis on which Article 28 of the VCLT may be applied by the Panel. The United States maintains that Article 28 can only be given effect as a rule of interpretation through Article 31.3(c) of the VCLT, while the European Communities appears to consider this approach too narrow and suggests that Article 28 of the VCLT may be given effect as a general principle of international law, independently of Article 31.3(c) of the VCLT. In our view, it is unnecessary to engage in this debate, as neither party disputes that the interpretation of Article 5 of the SCM Agreement should be consistent with the principle of nonretroactivity embodied in Article 28 of the VCLT. We therefore have made revisions to paragraph 7.46 and footnote 1536 (paragraph numbering unchanged, now footnote 1829) to clarify that we interpret Article 5 of the SCM Agreement consistently with the principle of non-retroactivity embodied in Article 28 of the VCLT, in accordance with the approach taken by the Appellate Body in prior disputes.
More on the same issue in paras. 6.23-25. Also, there's discussion of "applicable law" and the role of international law at paras. 7.70-105.
Clearly, international law will play a role in WTO dispute settlement. But as Joel, Joost and others can attest to, there is disagreement on the reasons for that and the extent of any such role.
