Fully fixing bug #3491 (anomaly when building _rect scheme in the

presence of let-ins and recursively non-uniform parameters).

The bug was in the List.chop of Inductiveops.get_arity which was wrong
in the presence of let-ins and recursively non-uniform parameters.

The bug #3491 showed up because, in addition to have let-ins, it was
wrongly detected as having recursively non-uniform parameters.

Also added some comments in declarations.mli.

2 files changed, 17 insertions, 9 deletions

diff --git a/pretyping/inductiveops.ml b/pretyping/inductiveops.ml
index 7f6a4a644..dfdc24d48 100644
--- a/pretyping/inductiveops.ml
+++ b/pretyping/inductiveops.ml
@@ -365,14 +365,16 @@ let get_arity env ((ind,u),params) =
   let (mib,mip) = Inductive.lookup_mind_specif env ind in
   let parsign =
     (* Dynamically detect if called with an instance of recursively
-       uniform parameter only or also of non recursively uniform
+       uniform parameter only or also of recursively non-uniform
        parameters *)
-    let parsign = mib.mind_params_ctxt in
-    let nnonrecparams = mib.mind_nparams - mib.mind_nparams_rec in
-    if Int.equal (List.length params) (rel_context_nhyps parsign - nnonrecparams) then
-      snd (List.chop nnonrecparams mib.mind_params_ctxt)
-    else
-      parsign in
+    let nparams = List.length params in
+    if Int.equal nparams mib.mind_nparams then
+      mib.mind_params_ctxt
+    else begin
+      assert (Int.equal nparams mib.mind_nparams_rec);
+      let nnonrecparamdecls = List.length mib.mind_params_ctxt - mib.mind_nparams_rec in
+      snd (List.chop nnonrecparamdecls mib.mind_params_ctxt)
+    end in
   let parsign = Vars.subst_instance_context u parsign in
   let arproperlength = List.length mip.mind_arity_ctxt - List.length parsign in
   let arsign,_ = List.chop arproperlength mip.mind_arity_ctxt in
diff --git a/pretyping/inductiveops.mli b/pretyping/inductiveops.mli
index af1783b70..7959759a3 100644
--- a/pretyping/inductiveops.mli
+++ b/pretyping/inductiveops.mli
@@ -25,7 +25,9 @@ val type_of_constructors : env -> pinductive -> types array
 (** Return constructor types in normal form *)
 val arities_of_constructors : env -> pinductive -> types array
 
-(** An inductive type with its parameters *)
+(** An inductive type with its parameters (transparently supports
+    reasoning either with only recursively uniform parameters or with all
+    parameters including the recursively non-uniform ones *)
 type inductive_family
 val make_ind_family : inductive puniverses * constr list -> inductive_family
 val dest_ind_family : inductive_family -> inductive puniverses * constr list
@@ -138,10 +140,14 @@ val lift_constructor : int -> constructor_summary -> constructor_summary
 val get_constructor :
   pinductive * mutual_inductive_body * one_inductive_body * constr list ->
   int -> constructor_summary
-val get_arity        : env -> inductive_family -> rel_context * sorts_family
 val get_constructors : env -> inductive_family -> constructor_summary array
 val get_projections  : env -> inductive_family -> constant array option
 
+(** [get_arity] returns the arity of the inductive family instantiated
+    with the parameters; if recursively non-uniform parameters are not
+    part of the inductive family, they appears in the arity *)
+val get_arity        : env -> inductive_family -> rel_context * sorts_family
+
 val build_dependent_constructor : constructor_summary -> constr
 val build_dependent_inductive   : env -> inductive_family -> constr
 val make_arity_signature : env -> bool -> inductive_family -> rel_context