Injection: do not fail when arguments have sort Prop.

This historic limitation of the injection tactic does not seem to have any
precise justification. In fact, the only equalities that are not projectable is
when the arguments of the constructor have sort Set or Type and the inductive
type is in Prop (due to the restriction on pattern matching).

The command "Unset Injection On Proofs" restores the old behavior.

1 files changed, 16 insertions, 2 deletions

diff --git a/tactics/equality.ml b/tactics/equality.ml
index 89c6a091a1..7b02443cae 100644
--- a/tactics/equality.ml
+++ b/tactics/equality.ml
@@ -610,6 +610,18 @@ let replace_in_clause_maybe_by c2 c1 cl tac_opt =
 exception DiscrFound of
   (constructor * int) list * constructor * constructor
 
+let injection_on_proofs = ref true
+
+let _ =
+  declare_bool_option
+    { optsync  = true;
+      optdepr  = false;
+      optname  = "injection on prop arguments";
+      optkey   = ["Injection";"On";"Proofs"];
+      optread  = (fun () -> !injection_on_proofs) ;
+      optwrite = (fun b -> injection_on_proofs := b) }
+
+
 let find_positions env sigma t1 t2 =
   let rec findrec sorts posn t1 t2 =
     let hd1,args1 = whd_betadeltaiota_stack env sigma t1 in
@@ -648,8 +660,10 @@ let find_positions env sigma t1 t2 =
             then [(List.rev posn,t1_0,t2_0)] else []
   in
   try
-    (* Rem: to allow injection on proofs objects, just add InProp *)
-    Inr (findrec [InSet;InType] [] t1 t2)
+    let sorts = if !injection_on_proofs then [InSet;InType;InProp]
+		else [InSet;InType]
+    in
+    Inr (findrec sorts [] t1 t2)
   with DiscrFound (path,c1,c2) ->
     Inl (path,c1,c2)