Constraint intersection

src/poetry/core/packages/constraints/union_constraint.py

@@ -1,5 +1,4 @@
 from typing import Tuple
-from typing import Union
 from typing import cast
 
 from poetry.core.packages.constraints.base_constraint import BaseConstraint
@@ -78,19 +77,43 @@ def intersect(self, other: "BaseConstraint") -> "BaseConstraint":
 
             return EmptyConstraint()
 
-        other = cast(Union[MultiConstraint, UnionConstraint], other)
-
+        # Two remaining cases: an intersection with another union, or an intersection
+        # with a multi.
+        #
+        # In the first case:
+        # (A or B) and (C or D) => (A and C) or (A and D) or (B and C) or (B and D)
+        #
+        # In the second case:
+        # (A or B) and (C and D) => (A and C and D) or (B and C and D)
         new_constraints = []
-        for our_constraint in self._constraints:
-            for their_constraint in other.constraints:
-                intersection = our_constraint.intersect(their_constraint)
+        if isinstance(other, UnionConstraint):
+            for our_constraint in self._constraints:
+                for their_constraint in other.constraints:
+                    intersection = our_constraint.intersect(their_constraint)
+
+                    if (
+                        not intersection.is_empty()
+                        and intersection not in new_constraints
+                    ):
+                        new_constraints.append(intersection)
+
+        else:
+            other = cast(MultiConstraint, other)
+
+            for our_constraint in self._constraints:
+                intersection = our_constraint
+                for their_constraint in other.constraints:
+                    intersection = intersection.intersect(their_constraint)
 
                 if not intersection.is_empty() and intersection not in new_constraints:
                     new_constraints.append(intersection)
 
         if not new_constraints:
             return EmptyConstraint()
 
+        if len(new_constraints) == 1:
+            return new_constraints[0]
+
         return UnionConstraint(*new_constraints)
 
     def union(self, other: BaseConstraint) -> "UnionConstraint":

tests/packages/constraints/test_constraint.py

@@ -88,6 +88,16 @@ def test_allows_all():
             Constraint("linux", "!="),
             MultiConstraint(Constraint("win32", "!="), Constraint("linux", "!=")),
         ),
+        (
+            UnionConstraint(Constraint("win32"), Constraint("linux")),
+            UnionConstraint(Constraint("win32"), Constraint("darwin")),
+            Constraint("win32"),
+        ),
+        (
+            UnionConstraint(Constraint("win32"), Constraint("linux")),
+            MultiConstraint(Constraint("win32", "!="), Constraint("darwin", "!=")),
+            Constraint("linux"),
+        ),
     ],
 )
 def test_intersect(