Correction du bug des types singletons pas sous-type de Set

(i.e. "Inductive unit := tt." conduisait à "t:Prop" alors que le
principe de la hiérarchie d'univers est d'être cumulative -- et que
Set en soit le niveau 0).

Une solution aurait été de poser Prop <= Set mais on adopte une autre
solution. Pour éviter le côté contre-intuitif d'avoir unit dans Type
et Prop <= Set, on garde la représentation de Prop au sein de la
hiérarchie prédicative sous la forme "Type (max ([],[])" (le niveau
sans aucune contrainte inférieure, appelons Type -1) et on adapte les
fonctions de sous-typage et de typage pour qu'elle prenne en compte la
règle Type -1 <= Prop (cf reduction.ml, reductionops.ml, et effets
incidents dans Termops.refresh_universes et Univ.super).

Petite uniformisation des noms d'univers et de sortes au passage
(univ.ml, univ.mli, term.ml, term.mli et les autres fichiers).



git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@10859 85f007b7-540e-0410-9357-904b9bb8a0f7

1 files changed, 22 insertions, 4 deletions

diff --git a/kernel/reduction.ml b/kernel/reduction.ml
index ba7e8c41d..ee93ec291 100644
--- a/kernel/reduction.ml
+++ b/kernel/reduction.ml
@@ -151,6 +151,25 @@ let compare_stacks f fmind lft1 stk1 lft2 stk2 cuniv =
 
 (* Convertibility of sorts *)
 
+(* The sort cumulativity is
+
+    Prop, Set <= Type 1 <= ... <= Type i <= ...
+
+    and this holds whatever Set is predicative or impredicative
+
+    In addition, there are the following internal sorts:
+    - Type (-1) is semantically equivalent to Prop but with the
+      following syntactic restrictions: 
+      Type (-1) is syntactically predicative and subtype of all Type i
+      Prop is syntactically impredicative and subtype of Type i only for i>=1
+    - Type 0 is semantically equivalent to predicative Set
+    Both Type (-1) and Type 0 can only occur as types of polymorphic 
+    inductive types (in practice Type 0 is systematically converted to Set and
+    we do not see it outside the computation of polymorphic inductive but
+    Type (-1) is kept to avoid setting Prop <= predicative Set in the syntax;
+    this means that (*1*) below can happen).
+*)
+
 type conv_pb = 
   | CONV 
   | CUMUL
@@ -158,14 +177,13 @@ type conv_pb =
 let sort_cmp pb s0 s1 cuniv =
   match (s0,s1) with
     | (Prop c1, Prop c2) -> if c1 = c2 then cuniv else raise NotConvertible
-    | (Prop c1, Type u)  ->
-        (match pb with
-            CUMUL -> cuniv
-          | _ -> raise NotConvertible)
+    | (Prop c1, Type u) when pb = CUMUL -> assert (is_univ_variable u); cuniv
     | (Type u1, Type u2) ->
+	assert (is_univ_variable u2);
 	(match pb with
            | CONV -> enforce_eq u1 u2 cuniv
 	   | CUMUL -> enforce_geq u2 u1 cuniv)
+    | (Type u, Prop _) when u = lower_univ & pb = CUMUL -> cuniv    (*1*)
     | (_, _) -> raise NotConvertible