Extraction: forbid Prop-polymorphism of inductives when extracting to Ocaml

 A particular case in sort-polymorphism of inductive types allows
 an informative type (such as prod) to have instances in Prop:

   (I,I) : True * True : Prop

 This is due to the fact that prod is a singleton type: indeed (I,I)
 has no informative content. But this invalidates an important invariant
 for the correctness of the extraction: inductive constructors stop
 having always the same sort as their inductive type. Consider for instance:

  Definition f (X:Type)(x:X*X)(g:X->nat) := g (fst x).
  Definition test := f _ (I,I) (fun _ => 0).

 Then the inductive element (I,I) is extracted as a logical part __,
 but during a strict evaluation (i.e. in Ocaml, not Haskell), this __
 will be given to fst, and hence to a match, leading to a nasty result
 (potentially segfault). Haskell is not affected, since fst is never
 evaluated.

 This patch adds a check for this situation during any Ocaml extraction,
 leading for the moment to a fatal error. Some functions in inductive.ml
 and retyping.ml now have an extra optional argument ?(polyprop=true)
 that should stay untouched in regular Coq usage, while type-checking
 done during extraction will disable this prop-polymorphism.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@14256 85f007b7-540e-0410-9357-904b9bb8a0f7

1 files changed, 1 insertions, 0 deletions

diff --git a/plugins/extraction/table.mli b/plugins/extraction/table.mli
index ce57a7840..d656edfb7 100644
--- a/plugins/extraction/table.mli
+++ b/plugins/extraction/table.mli
@@ -29,6 +29,7 @@ val error_inductive : global_reference -> 'a
 val error_nb_cons : unit -> 'a
 val error_module_clash : module_path -> module_path -> 'a
 val error_no_module_expr : module_path -> 'a
+val error_singleton_become_prop : identifier -> 'a
 val error_unknown_module : qualid -> 'a
 val error_scheme : unit -> 'a
 val error_not_visible : global_reference -> 'a