constraints/markers: fix union of multi constraints and thereby union of atomic multi markers resulting in invalid marker strings (#875)

Author: radoering

src/poetry/core/constraints/generic/multi_constraint.py

@@ -87,6 +87,11 @@ def union(self, other: BaseConstraint) -> BaseConstraint:
         if isinstance(other, MultiConstraint):
             theirs = set(other.constraints)
             common = [c for c in self.constraints if c in theirs]
+            if not common:
+                return AnyConstraint()
+            if len(common) == 1:
+                return common[0]
+
             return self.__class__(*common)
 
         if not isinstance(other, Constraint):

tests/constraints/generic/test_constraint.py

@@ -1152,7 +1152,12 @@ def test_intersect_extra(
         (
             MultiConstraint(Constraint("win32", "!="), Constraint("linux", "!=")),
             MultiConstraint(Constraint("win32", "!="), Constraint("darwin", "!=")),
-            MultiConstraint(Constraint("win32", "!=")),
+            Constraint("win32", "!="),
+        ),
+        (
+            MultiConstraint(Constraint("win32", "!="), Constraint("linux", "!=")),
+            MultiConstraint(Constraint("bsd", "!="), Constraint("darwin", "!=")),
+            AnyConstraint(),
         ),
         (
             Constraint("tegra", "not in"),

tests/version/test_markers.py

@@ -1910,6 +1910,27 @@ def test_partially_inverse_atomic_markers() -> None:
     assert str(m2.union(m1)) == 'sys_platform != "darwin"'
 
 
+def test_atomic_marker_union_is_any() -> None:
+    m1 = parse_marker('platform_machine != "arm64" and platform_machine != "aarch64"')
+    m2 = parse_marker('platform_machine != "darwin" and platform_machine != "x86_64"')
+
+    assert m1.union(m2).is_any()
+    assert m2.union(m1).is_any()
+
+
+def test_atomic_marker_union_is_single() -> None:
+    m1 = parse_marker('platform_machine != "arm64" and platform_machine != "aarch64"')
+    m2 = parse_marker('platform_machine != "arm64" and platform_machine != "x86_64"')
+
+    result1 = m1.union(m2)
+    assert str(result1) == 'platform_machine != "arm64"'
+    assert isinstance(result1, SingleMarker)
+
+    result2 = m2.union(m1)
+    assert str(result2) == 'platform_machine != "arm64"'
+    assert isinstance(result2, SingleMarker)
+
+
 @pytest.mark.parametrize(
     "scheme, marker, expected",
     [