Add a test for desired output for solver messaging

tests/test_resolve.py

@@ -420,6 +420,22 @@ def test_unsat_shortest_chain_3():
     assert "c=1.3.6" in str(excinfo.value)
 
 
+def test_unsat_shortest_chain_4():
+    index = (
+        simple_rec(name='a', depends=['py =3.7.1']),
+        simple_rec(name='b', depends=['py =3.6.1']),
+        simple_rec(name="py_req_1", version="1.0.0"),
+        simple_rec(name="py_req_2", version="1.0.0"),
+        simple_rec(name='py', version='3.7.1', depends=['py_req_1', 'py_req_2']),
+        simple_rec(name='py', version='3.6.1', depends=['pr_req_1', 'py_req_2']),
+    )
+    r = Resolve(OrderedDict((prec, prec) for prec in index))
+    with pytest.raises(UnsatisfiableError) as excinfo:
+        r.install(['a', 'py=3.6.1'])
+    assert "a -> py=3.7.1" in str(excinfo.value)
+    assert "py=3.6.1" in str(excinfo.value)
+    assert "py=3.6.1 -> py_req_2" not in str(excinfo.value)
+
 def test_unsat_chain():
     # a -> b -> c=1.x -> d=1.x
     # e      -> c=2.x -> d=2.x