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, 10 insertions, 1 deletions

diff --git a/kernel/inductive.mli b/kernel/inductive.mli
index c14f69343..a124647c6 100644
--- a/kernel/inductive.mli
+++ b/kernel/inductive.mli
@@ -82,7 +82,16 @@ val check_cofix : env -> cofixpoint -> unit
 
 (** {6 Support for sort-polymorphic inductive types } *)
 
-val type_of_inductive_knowing_parameters :
+(** The "polyprop" optional argument below allows to control
+    the "Prop-polymorphism". By default, it is allowed.
+    But when "polyprop=false", the following exception is raised
+    when a polymorphic singleton inductive type becomes Prop due to
+    parameter instantiation. This is used by the Ocaml extraction,
+    which cannot handle (yet?) Prop-polymorphism. *)
+
+exception SingletonInductiveBecomesProp of identifier
+
+val type_of_inductive_knowing_parameters : ?polyprop:bool ->
   env -> one_inductive_body -> types array -> types
 
 val max_inductive_sort : sorts array -> universe