Tracing ambiguity in GADT type inference

GADTs, short for Generalized Algebraic DataTypes, extend usual algebraic datatypes with a form of dependent typing that has many useful applications, but raises serious issues for type inference. Pattern matching on GADTs introduces type equalities with limited scopes, which are a source of ambiguities that may destroy principal types—and must be resolved by type annotations. By tracing ambiguities in types, we may tighten the definition of ambiguities and confine them, so as to request fewer type annotations. Now in use in OCaml 4.00, this solution also lifts some restrictions on object and polymorphic variant types that appeared in a previous implementation of GADTs in OCaml.