2 files changed, 47 insertions, 0 deletions

diff --git a/test-suite/output/Error_msg_diffs.out b/test-suite/output/Error_msg_diffs.out
new file mode 100644
index 0000000000..3e337e892d
--- /dev/null
+++ b/test-suite/output/Error_msg_diffs.out
@@ -0,0 +1,12 @@
+File "stdin", line 32, characters 0-12:
+Error:
+In environment
+T : Type
+p : T -> bool
+a : T
+t1, t2 : btree T
+IH1 : count p (rev_tree t1) = count p t1
+IH2 : count p (rev_tree t2) = count p t2
+Unable to unify "(if p a then 1 else 0) + (count p t1 + count p t2)" with
+ "(if p a then 1 else 0) + (count p t2 + count p t1)".
+
diff --git a/test-suite/output/Error_msg_diffs.v b/test-suite/output/Error_msg_diffs.v
new file mode 100644
index 0000000000..11c766b210
--- /dev/null
+++ b/test-suite/output/Error_msg_diffs.v
@@ -0,0 +1,35 @@
+(* coq-prog-args: ("-color" "on" "-async-proofs" "off") *)
+(* Re: -async-proofs off, see https://github.com/coq/coq/issues/9671 *)
+(* Shows diffs in an error message for an "Unable to unify" error *)
+Require Import Arith List Bool.
+
+Inductive btree (T : Type) : Type :=
+  Leaf | Node (val : T) (t1 t2 : btree T).
+
+Arguments Leaf {T}.
+Arguments Node {T}.
+
+Fixpoint rev_tree {T : Type} (t : btree T) : btree T :=
+match t with
+| Leaf => Leaf
+| Node x t1 t2 => Node x (rev_tree t2) (rev_tree t1)
+end.
+
+Fixpoint count {T : Type} (p : T -> bool) (t : btree T) : nat :=
+match t with
+| Leaf => 0
+| Node x t1 t2 =>
+  (if p x then 1 else 0) + (count p t1 + count p t2)
+end.
+
+Lemma count_rev_tree {T} (p : T -> bool) t : count p (rev_tree t) = count p t.
+Proof.
+induction t as [ | a t1 IH1 t2 IH2].
+  easy.
+simpl.
+rewrite IH1.
+rewrite IH2.
+reflexivity.
+rewrite (Nat.add_comm (count p t2)).
+easy.
+Qed.