It depends which court is deciding:

1. International Court- will uphold treaty obligation in general.

2. Domestic Court- will uphold local laws.

Specific Situatiions of Conflict:

1. Treaty vs. Constitution- A46, VCLT, when constitutional violation is manifest and concerns a rule of internal law of fundamental importance, state may deviate from treaty obligation. (Note: "A party may not invoke the provisions of internal law as justification for its failure to  perform a treaty." Under the dualist theory, unconstitutionality of a treaty is purely domestic matter. State faces risk of international sanction.)

2. Treaty vs. Domestic legislation- when the two instruments relate to the same subject, try to give effect to both; if inconsistent, the later in date will control, provided that the treaty stipulation is self- executing. But the  rule applies in domestic sphere. A treaty, even if contrary to a later statute, is binding in international law.