From d0e258175280152e504a296a05ecbd1920bb17c9 Mon Sep 17 00:00:00 2001
From: Gaëtan Gilbert
Date: Tue, 14 May 2019 13:10:20 +0200
Subject: Fix #10161: occur check when defining an algebraic universe.

---
 engine/uState.ml                   | 3 ++-
 test-suite/bugs/closed/bug_10161.v | 8 ++++++++
 2 files changed, 10 insertions(+), 1 deletion(-)
 create mode 100644 test-suite/bugs/closed/bug_10161.v

diff --git a/engine/uState.ml b/engine/uState.ml
index adea78d4c9..9b27ff7916 100644
--- a/engine/uState.ml
+++ b/engine/uState.ml
@@ -214,7 +214,8 @@ let process_universe_constraints ctx cstrs =
   | Inr l, Inl r | Inl r, Inr l ->
     let alg = LSet.mem l ctx.uctx_univ_algebraic in
     let inst = univ_level_rem l r r in
-      if alg then (instantiate_variable l inst vars; local)
+      if alg && not (LSet.mem l (Universe.levels inst)) then
+        (instantiate_variable l inst vars; local)
       else
         let lu = Universe.make l in
         if univ_level_mem l r then
diff --git a/test-suite/bugs/closed/bug_10161.v b/test-suite/bugs/closed/bug_10161.v
new file mode 100644
index 0000000000..3d262b89fe
--- /dev/null
+++ b/test-suite/bugs/closed/bug_10161.v
@@ -0,0 +1,8 @@
+Inductive SwitchT (A : Type) : Type :=
+| switchT : forall T, SwitchT T -> SwitchT A.
+
+Set Printing Universes.
+
+Fail Inductive UseSwitchT :=
+| useSwitchT : SwitchT UseSwitchT -> UseSwitchT.
+(* used to stack overflow, should be univ inconsistency cannot satisfy u = u+1 *)
-- 
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