Fix handling of root incompatibilities

poetry/mixology/failure.py

@@ -8,8 +8,6 @@ from .incompatibility_cause import ConflictCause
 
 class SolveFailure(Exception):
     def __init__(self, incompatibility):  # type: (Incompatibility) -> None
-        assert incompatibility.terms[0].dependency.is_root
-
         self._incompatibility = incompatibility
 
     @property

tests/mixology/version_solver/test_unsolvable.py

@@ -51,6 +51,20 @@ Because bar (1.0.0) depends on shared (>3.0.0)
 So, because myapp depends on both foo (1.0.0) and bar (1.0.0), version solving failed."""
 
     check_solver_result(root, provider, error=error)
+    check_solver_result(root, provider, error=error)
+
+
+def test_disjoint_root_constraints(root, provider, repo):
+    root.add_dependency("foo", "1.0.0")
+    root.add_dependency("foo", "2.0.0")
+
+    add_to_repo(repo, "foo", "1.0.0")
+    add_to_repo(repo, "foo", "2.0.0")
+
+    error = """\
+Because myapp depends on both foo (1.0.0) and foo (2.0.0), version solving failed."""
+
+    check_solver_result(root, provider, error=error)
 
 
 def test_no_valid_solution(root, provider, repo):