bug in bigm transformation with set objects

[michaelbynum]

Summary/Motivation:

When creating a constraint using the index set of another constraint combined with another set, the index set of the original constraint gets modified unintentionally. See the following example:

>>> import pyomo.environ as pe
>>> m = pe.ConcreteModel()
>>> m.a = pe.Set(initialize=[1,2,3])
>>> m.b = pe.Set(initialize=['a', 'b'])
>>> m.c = pe.Constraint(m.a, m.b)
>>> m.c.index_set().pprint()
c_index : Dim=0, Dimen=2, Size=6, Domain=None, Ordered=False, Bounds=None
    Virtual
>>> for i in m.c.index_set():
...     print(i)
...
(1, 'a')
(1, 'b')
(2, 'a')
(2, 'b')
(3, 'a')
(3, 'b')
>>> m.c2 = pe.Constraint(m.c.index_set(), m.b)
>>> m.c.index_set().pprint()
c_index : Dim=0, Dimen=2, Size=12, Domain=None, Ordered=False, Bounds=None
    Virtual
>>> for i in m.c.index_set():
...     print(i)
...
(1, 'a', 'a')
(1, 'a', 'b')
(1, 'b', 'a')
(1, 'b', 'b')
(2, 'a', 'a')
(2, 'a', 'b')
(2, 'b', 'a')
(2, 'b', 'b')
(3, 'a', 'a')
(3, 'a', 'b')
(3, 'b', 'a')
(3, 'b', 'b')

This creates a bug in the BigM transformation in pyomo.gdp. This PR fixes this problem by wrapping the original index set in a list() when creating the new constraint.

This is may not be the correct fix. We may need a fix in the Constraint constructor or somewhere in Sets. However, the new Set rewrite may fix this problem. @jsiirola What is your suggestion on how to proceed? Is this the correct fix until the set rewrite is complete or should I go ahead and try to fix the underlying problem?

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[jsiirola]

This is a duplicate of #134. The set-rewrite (#326) does indeed fix this bug. While this technically works around #134, the better solution is to fix _SetProduct - which now that I look into it, I don't think is a bad patch:

diff --git a/pyomo/core/base/sets.py b/pyomo/core/base/sets.py
index 4e27e98..1fe17af 100644
--- a/pyomo/core/base/sets.py
+++ b/pyomo/core/base/sets.py
@@ -1545,7 +1545,7 @@ class _SetProduct(_SetOperator):
         _SetOperator.__init__(self, *args, **kwd)
         # the individual index sets definining the product set.
         if isinstance(self._setA,_SetProduct):
-            self.set_tuple = self._setA.set_tuple
+            self.set_tuple = list(self._setA.set_tuple)
         else:
             self.set_tuple = [self._setA]
         if isinstance(self._setB,_SetProduct):