Fix Version.difference with a local variant of the same version

src/poetry/core/constraints/version/version.py

@@ -121,10 +121,23 @@ def union(self, other: VersionConstraint) -> VersionConstraint:
 
         return VersionUnion.of(self, other)
 
-    def difference(self, other: VersionConstraint) -> Version | EmptyConstraint:
+    def difference(self, other: VersionConstraint) -> VersionConstraint:
         if other.allows(self):
             return EmptyConstraint()
 
+        if isinstance(other, Version) and self.allows(other):
+            # `self` is a non-local version and `other` a local variant of it,
+            # e.g. `2.12.1`.difference(`2.12.1+cpu`). The result must not
+            # allow `other`, which a plain `Version` cannot express, so
+            # delegate to the equivalent single-point range whose difference
+            # arithmetic can represent the split.
+            # See https://github.com/python-poetry/poetry/issues/10965.
+            from poetry.core.constraints.version.version_range import VersionRange
+
+            return VersionRange(
+                self, self, include_min=True, include_max=True
+            ).difference(other)
+
         return self
 
     def flatten(self) -> list[VersionRangeConstraint]:

tests/constraints/version/test_version.py

@@ -518,6 +518,25 @@ def test_difference() -> None:
     )
 
 
+def test_difference_local_version() -> None:
+    """The difference of a non-local version and one of its local variants
+    must not allow the subtracted local variant.
+
+    Regression test for https://github.com/python-poetry/poetry/issues/10965
+    (the solver looped forever because the difference made no progress).
+    """
+    public = Version.parse("2.12.1")
+    local = Version.parse("2.12.1+cpu")
+
+    difference = public.difference(local)
+
+    assert not difference.allows(local)
+    assert difference.allows(public)
+    # The reverse direction is unchanged: a non-local version constraint
+    # subsumes its local variants entirely.
+    assert local.difference(public).is_empty()
+
+
 @pytest.mark.parametrize(
     "version,normalized_version",
     [