Add explanatory comment and tests

src/poetry/core/packages/dependency.py

@@ -574,6 +574,11 @@ def __eq__(self, other: object) -> bool:
         if not isinstance(other, Dependency):
             return NotImplemented
 
+        # "constraint" is implicitly given for direct origin dependencies and might not
+        # be set yet ("*"). Thus, it shouldn't be used to determine if two direct origin
+        # dependencies are equal.
+        # Calling is_direct_origin() for one dependency is sufficient because
+        # super().__eq__() returns False for different origins.
         return super().__eq__(other) and (
             self._constraint == other.constraint or self.is_direct_origin()
         )

tests/packages/test_dependency.py

@@ -361,6 +361,34 @@ def test_create_from_pep_508_url_with_activated_extras() -> None:
     assert dependency.extras == {"fred", "bar"}
 
 
+@pytest.mark.parametrize(
+    "dependency1, dependency2, expected",
+    [
+        (Dependency("a", "1.0"), Dependency("a", "1.0"), True),
+        (Dependency("a", "1.0"), Dependency("a", "1.0.1"), False),
+        (Dependency("a", "1.0"), Dependency("a1", "1.0"), False),
+        (Dependency("a", "1.0"), Dependency("a", "1.0", source_type="file"), False),
+        # constraint is implicitly given for direct origin dependencies,
+        # but might not be set
+        (
+            Dependency("a", "1.0", source_type="file"),
+            Dependency("a", "*", source_type="file"),
+            True,
+        ),
+        # constraint is not implicit for non direct origin dependencies
+        (Dependency("a", "1.0"), Dependency("a", "*"), False),
+        (
+            Dependency("a", "1.0", source_type="legacy"),
+            Dependency("a", "*", source_type="legacy"),
+            False,
+        ),
+    ],
+)
+def test_eq(dependency1: Dependency, dependency2: Dependency, expected: bool) -> None:
+    assert (dependency1 == dependency2) is expected
+    assert (dependency2 == dependency1) is expected
+
+
 @pytest.mark.parametrize(
     "attr_name, value",
     [