deps: support valid PEP 517 projects as dir deps

src/poetry/core/packages/directory_dependency.py

@@ -36,7 +36,6 @@ def __init__(
                 raise ValueError(f"Directory {self._path} does not exist")
 
         self._develop = develop
-        self._supports_poetry = False
 
         if not self._full_path.exists():
             raise ValueError(f"Directory {self._path} does not exist")
@@ -45,12 +44,17 @@ def __init__(
             raise ValueError(f"{self._path} is a file, expected a directory")
 
         # Checking content to determine actions
-        setup = self._full_path / "setup.py"
-        self._supports_poetry = PyProjectTOML(
-            self._full_path / "pyproject.toml"
-        ).is_poetry_project()
+        setup_py = self._full_path / "setup.py"
+        setup_cfg = self._full_path / "setup.cfg"
+        setuptools_project = setup_py.exists() or setup_cfg.exists()
+        pyproject = PyProjectTOML(self._full_path / "pyproject.toml")
 
-        if not setup.exists() and not self._supports_poetry:
+        self._supports_pep517 = (
+            setuptools_project or pyproject.is_build_system_defined()
+        )
+        self._supports_poetry = pyproject.is_poetry_project()
+
+        if not (self._supports_pep517 or self._supports_poetry):
             raise ValueError(
                 f"Directory {self._full_path} does not seem to be a Python package"
             )

src/poetry/core/pyproject/toml.py

@@ -40,6 +40,9 @@ def data(self) -> TOMLDocument:
 
         return self._data
 
+    def is_build_system_defined(self) -> bool:
+        return self._file.exists() and "build-system" in self.data
+
     @property
     def build_system(self) -> BuildSystem:
         from poetry.core.pyproject.tables import BuildSystem

tests/fixtures/project_with_pep517_non_poetry/pyproject.toml

@@ -0,0 +1,13 @@
+[build-system]
+requires = ["flit_core >=3.7.1,<4"]
+build-backend = "flit_core.buildapi"
+
+[project]
+name = "flit"
+authors = []
+dependencies = [
+    "flit_core >=3.7.1",
+]
+requires-python = ">=3.6"
+readme = "README.rst"
+dynamic = ['version', 'description']

tests/fixtures/project_with_setup_cfg_only/setup.cfg

@@ -0,0 +1,18 @@
+[metadata]
+name = my_package
+version = attr: my_package.VERSION
+description = My package description
+long_description = file: README.rst, CHANGELOG.rst, LICENSE.rst
+keywords = one, two
+license = BSD 3-Clause License
+classifiers =
+    Framework :: Django
+    Programming Language :: Python :: 3
+
+[options]
+zip_safe = False
+include_package_data = True
+packages = find:
+install_requires =
+    requests
+    importlib-metadata; python_version<"3.8"

tests/packages/test_directory_dependency.py

@@ -75,3 +75,21 @@ def test_directory_dependency_pep_508_extras() -> None:
     requirement = f"demo[foo,bar] @ file://{path.as_posix()}"
     requirement_expected = f"demo[bar,foo] @ file://{path.as_posix()}"
     _test_directory_dependency_pep_508("demo", path, requirement, requirement_expected)
+
+
+@pytest.mark.parametrize(
+    ("fixture", "name"),
+    [
+        ("project_with_pep517_non_poetry", "PEP 517"),
+        ("project_with_setup_cfg_only", "setup.cfg"),
+    ],
+)
+def test_directory_dependency_non_poetry_pep517(fixture: str, name: str) -> None:
+    path = Path(__file__).parent.parent / "fixtures" / fixture
+
+    try:
+        DirectoryDependency("package", path)
+    except ValueError as e:
+        if "does not seem to be a Python package" not in str(e):
+            raise e from e
+        pytest.fail(f"A {name} project not recognized as valid directory dependency")