From dd768a0c3d8c3cf24c3aaf383ddabe67a9acb408 Mon Sep 17 00:00:00 2001
From: Marcel Stimberg <marcel.stimberg@inserm.fr>
Date: Thu, 27 Aug 2020 18:05:43 +0200
Subject: [PATCH] Exclude "constant over dt" subexpressions from cycle checking

Fixes #1187
---
 brian2/equations/equations.py  | 14 +++++++++-----
 brian2/tests/test_equations.py | 14 ++++++++++++++
 2 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/brian2/equations/equations.py b/brian2/equations/equations.py
index d4b77c533..aa22af3f8 100644
--- a/brian2/equations/equations.py
+++ b/brian2/equations/equations.py
@@ -901,14 +901,18 @@ def _sort_subexpressions(self):
         static_deps = {}
         for eq in self._equations.values():
             if eq.type == SUBEXPRESSION:
-                static_deps[eq.varname] = [dep for dep in eq.identifiers if
-                                           dep in self._equations and
-                                           self._equations[dep].type == SUBEXPRESSION]
-        
+                # "Constant over dt" subexpressions are implemented like parameters
+                # and can be considered as not depending on anything
+                if 'constant over dt' in eq.flags:
+                    static_deps[eq.varname] = []
+                else:
+                    static_deps[eq.varname] = [dep for dep in eq.identifiers if
+                                               dep in self._equations and
+                                               self._equations[dep].type == SUBEXPRESSION]
         try:
             sorted_eqs = topsort(static_deps)
         except ValueError:
-            raise ValueError('Cannot resolve dependencies between static '
+            raise ValueError('Cannot resolve dependencies between '
                              'equations, dependencies contain a cycle.')
 
         # put the equations objects in the correct order
diff --git a/brian2/tests/test_equations.py b/brian2/tests/test_equations.py
index 0c614750c..eda780173 100644
--- a/brian2/tests/test_equations.py
+++ b/brian2/tests/test_equations.py
@@ -485,6 +485,20 @@ def test_extract_subexpressions():
     assert constant['s2'].type == SUBEXPRESSION
 
 
+@pytest.mark.codegen_independent
+def test_cyclical_subexpressions():
+    with pytest.raises(ValueError):
+        # dependency cycle
+        Equations('''dv/dt = (-v + s1)/ (10*ms) : 1
+                     s1 = 2 * s2 : 1
+                     s2 = s1/2 : 1''')
+
+    # With constant over dt, the cycle disappears
+    Equations('''dv/dt = (-v + s1)/ (10*ms) : 1
+                 s1 = 2 * s2 : 1
+                 s2 = s1/2 : 1 (constant over dt)''')
+
+
 @pytest.mark.codegen_independent
 def test_repeated_construction():
     eqs1 = Equations('dx/dt = x : 1')