Allow path dependency that is missing build-system

src/poetry/core/packages/dependency.py

@@ -406,8 +406,7 @@ def create_from_pep_508(
             if os.path.isdir(p) and (os.path.sep in name or name.startswith(".")):
                 if not is_python_project(Path(name)):
                     raise ValueError(
-                        f"Directory {name!r} is not installable. File 'setup.[py|cfg]' "
-                        "not found."
+                        f"Directory {name!r} is not installable. Not a Python project."
                     )
                 link = Link(path_to_url(p))
             elif is_archive_file(p):

src/poetry/core/packages/utils/utils.py

@@ -18,7 +18,6 @@
 from poetry.core.constraints.version import Version
 from poetry.core.constraints.version import VersionRange
 from poetry.core.constraints.version import parse_marker_version_constraint
-from poetry.core.pyproject.toml import PyProjectTOML
 from poetry.core.version.markers import SingleMarker
 from poetry.core.version.markers import SingleMarkerLike
 from poetry.core.version.markers import dnf
@@ -129,12 +128,9 @@ def is_python_project(path: Path) -> bool:
     setup_cfg = path / "setup.cfg"
     setuptools_project = setup_py.exists() or setup_cfg.exists()
 
-    pyproject = PyProjectTOML(path / "pyproject.toml")
+    pyproject = (path / "pyproject.toml").exists()
 
-    supports_pep517 = setuptools_project or pyproject.is_build_system_defined()
-    supports_poetry = pyproject.is_poetry_project()
-
-    return supports_pep517 or supports_poetry
+    return pyproject or setuptools_project
 
 
 def is_archive_file(name: str) -> bool:

src/poetry/core/pyproject/toml.py

@@ -33,9 +33,6 @@ def data(self) -> dict[str, Any]:
 
         return self._data
 
-    def is_build_system_defined(self) -> bool:
-        return "build-system" in self.data
-
     @property
     def build_system(self) -> BuildSystem:
         if self._build_system is None:

tests/fixtures/project_minimal/pyproject.toml

@@ -0,0 +1,3 @@
+[project]
+name = "minimal"
+version = "1"

tests/packages/test_directory_dependency.py

@@ -66,6 +66,12 @@ def test_directory_dependency_is_not_a_python_project(
         dep.validate(raise_error=True)
 
 
+def test_directory_dependency_minimal() -> None:
+    path = Path(__file__).parent.parent / "fixtures" / "project_minimal"
+    dep = DirectoryDependency("demo", path)
+    dep.validate(raise_error=True)
+
+
 def _test_directory_dependency_pep_508(
     name: str, path: Path, pep_508_input: str, pep_508_output: str | None = None
 ) -> None: