Camlp4 accepts bad syntax and may also bomb on it (Mantis 5705)

[damiendoligez]

Transferred from http://caml.inria.fr/mantis/view.php?id=5705
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The following file x.ml will lead to a syntax error with ocaml x.ml:

  let f x : type a. a = ()

But camlp4o x.ml will accept and print the human-readable format.

By contrast, ocamldep -pp camlp4o x.ml will bomb:

  Assertion failed, file "camlp4/Camlp4/Struct/Camlp4Ast2OCamlAst.ml", line 284, char 8
  Preprocessing error on file x.ml
  x.cmo :
  x.cmx :
  EXIT STATUS 2