Fix complicated export case

By treating different python version ranges independently, we buy the
flexibility needed to make better decisions.

Author: dimbleby

Diff for: src/poetry/packages/locker.py

@@ -19,8 +19,11 @@
 from poetry.core.packages.url_dependency import URLDependency
 from poetry.core.packages.vcs_dependency import VCSDependency
 from poetry.core.semver.helpers import parse_constraint
+from poetry.core.semver.util import constraint_regions
 from poetry.core.semver.version import Version
 from poetry.core.toml.file import TOMLFile
+from poetry.core.version.markers import AnyMarker
+from poetry.core.version.markers import SingleMarker
 from poetry.core.version.markers import parse_marker
 from poetry.core.version.requirements import InvalidRequirement
 from tomlkit import array
@@ -49,6 +52,30 @@
 logger = logging.getLogger(__name__)
 
 
+def get_python_version_region_markers(packages: list[Package]) -> list[BaseMarker]:
+    regions = constraint_regions([package.python_constraint for package in packages])
+
+    markers = []
+    for region in regions:
+        min_operator = ">=" if region.include_min else ">"
+        lo = (
+            SingleMarker("python_full_version", f"{min_operator} {region.min}")
+            if region.min is not None
+            else AnyMarker()
+        )
+
+        max_operator = "<=" if region.include_max else "<"
+        hi = (
+            SingleMarker("python_full_version", f"{max_operator} {region.max}")
+            if region.max is not None
+            else AnyMarker()
+        )
+
+        markers.append(lo.intersect(hi))
+
+    return markers
+
+
 class Locker:
 
     _VERSION = "1.1"
@@ -279,12 +306,25 @@ def __walk_dependencies(
                 ):
                     continue
 
-                require = deepcopy(require)
-                require.marker = require.marker.intersect(
+                base_marker = require.marker.intersect(
                     requirement.marker.without_extras()
                 )
-                if not require.marker.is_empty():
-                    dependencies.append(require)
+
+                if not base_marker.is_empty():
+                    # So as to give ourselves enough flexibility in choosing a solution,
+                    # we need to split the world up into the python version ranges that
+                    # this package might care about.
+                    #
+                    # We create a marker for all of the possible regions, and add a
+                    # requirement for each separately.
+                    candidates = packages_by_name.get(require.name, [])
+                    region_markers = get_python_version_region_markers(candidates)
+                    for region_marker in region_markers:
+                        marker = region_marker.intersect(base_marker)
+                        if not marker.is_empty():
+                            require2 = deepcopy(require)
+                            require2.marker = marker
+                            dependencies.append(require2)
 
             key = locked_package
             if key not in nested_dependencies: