Fixing an evar leak in pattern-matching compilation (#3758).

2 files changed, 23 insertions, 3 deletions

diff --git a/pretyping/cases.ml b/pretyping/cases.ml
index 78837b9d8c..7da59b94e2 100644
--- a/pretyping/cases.ml
+++ b/pretyping/cases.ml
@@ -293,10 +293,13 @@ let try_find_ind env sigma typ realnames =
   IsInd (typ,ind,names)
 
 let inh_coerce_to_ind evdref env ty tyi =
+  let sigma = !evdref in
   let expected_typ = inductive_template evdref env None tyi in
-     (* devrait être indifférent d'exiger leq ou pas puisque pour
-        un inductif cela doit être égal *)
-  let _ = e_cumul env evdref expected_typ ty in ()
+  (* Try to refine the type with inductive information coming from the
+     constructor and renounce if not able to give more information *)
+  (* devrait être indifférent d'exiger leq ou pas puisque pour
+     un inductif cela doit être égal *)
+  if not (e_cumul env evdref expected_typ ty) then evdref := sigma
 
 let binding_vars_of_inductive = function
   | NotInd _ -> []
diff --git a/test-suite/success/Cases-bug3758.v b/test-suite/success/Cases-bug3758.v
new file mode 100644
index 0000000000..e48f452326
--- /dev/null
+++ b/test-suite/success/Cases-bug3758.v
@@ -0,0 +1,17 @@
+(* There used to be an evar leak in the to_nat example *)
+
+Require Import Coq.Lists.List.
+Import ListNotations.
+
+Fixpoint Idx {A:Type} (l:list A) : Type :=
+  match l with
+  | [] => False
+  | _::l => True + Idx l
+  end.
+
+Fixpoint to_nat {A:Type} (l:list A) (i:Idx l) : nat :=
+  match l,i with
+  |  [] , i => match i with end
+  | _::_, inl _ => 0
+  | _::l, inr i => S (to_nat l i)
+  end.