Generalized algebraic datatypes, or GADTs, extend usual sum types in
two ways: constraints on type parameters may change depending on the
value constructor, and some type variables may be existentially
quantified. They are described in chapter \ref{c:gadts-tutorial}.

(Introduced in OCaml 4.00)

\begin{syntax}
constr-decl:
          ...
        | constr-name ':' [ constr-args '->' ] typexpr
;
type-param:
          ...
        | [variance] '_'
\end{syntax}

Refutation cases. (Introduced in OCaml 4.03)

\begin{syntax}
matching-case:
     pattern ['when' expr] '->' expr
   | pattern '->' '.'
\end{syntax}

Explicit naming of existentials. (Introduced in OCaml 4.13.0)

\begin{syntax}
pattern:
     ...
   | constr '(' "type" {{typeconstr-name}} ')' '(' pattern ')'
;
\end{syntax}