Fix circular dependencies resolution

CHANGELOG.md

 # Change Log
 
+## [Unreleased]
+
+### Fixed
+
+- Fixed dependency resolution with circular dependencies.
+
+
 ## [0.12.16] - 2019-05-17
 
 ### Fixed

poetry/puzzle/solver.py

@@ -248,7 +248,10 @@ class Solver:
                         break
 
             if previous and previous["name"] == dependency.name:
-                break
+                # We have a circular dependency.
+                # Since the dependencies are resolved we can
+                # simply skip it because we already have it
+                continue
 
             for pkg in packages:
                 if pkg.name == dependency.name and dependency.constraint.allows(

tests/puzzle/test_solver.py

@@ -726,20 +726,27 @@ def test_solver_circular_dependency(solver, repo, package):
 
     package_b = get_package("B", "1.0")
     package_b.add_dependency("A", "^1.0")
+    package_b.add_dependency("C", "^1.0")
+
+    package_c = get_package("C", "1.0")
 
     repo.add_package(package_a)
     repo.add_package(package_b)
+    repo.add_package(package_c)
 
     ops = solver.solve()
 
     check_solver_result(
         ops,
         [
+            {"job": "install", "package": package_c},
             {"job": "install", "package": package_b},
             {"job": "install", "package": package_a},
         ],
     )
 
+    assert "main" == ops[0].package.category
+
 
 def test_solver_duplicate_dependencies_same_constraint(solver, repo, package):
     package.add_dependency("A")