Subtyping: align coqtop behavior concerning opaque csts on coqchk's one

 After discussion with Bruno and Hugo, coqtop now accepts that an opaque
 constant in a module type could be implemented by anything of
 the right type, even if bodies differ. Said otherwise, with respect
 to subtyping, an opaque constant behaves just as a parameter.
 This was already the case in coqchk, and a footnote in documentation
 is advertising for quite some time that:

   "Opaque definitions are processed as assumptions."

 Truly, it might seem awkward that "Definition x:=3" can implement
 "Lemma x:nat. Proof 2. Qed." but the opacity ensures that nothing
 can go wrong afterwards, since Coq is forced to ignore that the x
 in signature has body "2".

 Similarly, "T with Definition x := c" is now legal when T contains
 an opaque x, even when this x isn't convertible with c.

 By avoiding accesses to opaque bodies, we also achieve some speedup
 (less delayed load of .vo final sections containing opaque terms).

 Nota: the extraction will have to be adapted, since for the moment it
 might access the body of opaque constants: the warning emitted when
 doing that should become an error.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@13987 85f007b7-540e-0410-9357-904b9bb8a0f7

1 files changed, 5 insertions, 3 deletions

diff --git a/checker/mod_checking.ml b/checker/mod_checking.ml
index e6582e918..d35dfaff1 100644
--- a/checker/mod_checking.ml
+++ b/checker/mod_checking.ml
@@ -110,8 +110,11 @@ let check_definition_sub env cb1 cb2 =
   let typ1 = Typeops.type_of_constant_type env cb1.const_type in
   let typ2 = Typeops.type_of_constant_type env cb2.const_type in
   check_type env typ1 typ2;
+  (* In the spirit of subtyping.check_constant, we accept
+     any implementations of parameters and opaques terms,
+     as long as they have the right type *)
   (match cb2.const_body with
-    | Undef _ -> ()
+    | Undef _ | OpaqueDef _ -> ()
     | Def lc2 ->
 	(match cb1.const_body with
 	  | Def lc1 ->
@@ -119,8 +122,7 @@ let check_definition_sub env cb1 cb2 =
             let c2 = force_constr lc2 in
 	    Reduction.conv env c1 c2
 	  (* Coq only places transparent cb in With_definition_body *)
-	  | _ -> assert false)
-    | _ -> ()) (* Pierre L: shouldn't this case raise an error ? *)
+	  | _ -> assert false))
 
 let lookup_modtype mp env =
   try Environ.lookup_modtype mp env