solver: fix a special case with unconditional dependency of an extra

src/poetry/puzzle/provider.py

@@ -20,6 +20,7 @@
 from poetry.core.constraints.version import Version
 from poetry.core.packages.utils.utils import get_python_constraint_from_marker
 from poetry.core.version.markers import AnyMarker
+from poetry.core.version.markers import EmptyMarker
 from poetry.core.version.markers import MarkerUnion
 
 from poetry.inspection.info import PackageInfo
@@ -946,20 +947,13 @@ def _merge_dependencies_by_constraint(
         for dep in dependencies:
             by_constraint[dep.constraint].append(dep)
         for constraint, _deps in by_constraint.items():
-            new_markers = []
-            for dep in _deps:
-                marker = dep.marker.without_extras()
-                if marker.is_any():
-                    # No marker or only extras
-                    continue
-
-                new_markers.append(marker)
-
-            if not new_markers:
-                continue
+            new_markers = [dep.marker for dep in _deps]
 
             dep = _deps[0]
-            dep.marker = dep.marker.union(MarkerUnion(*new_markers))
+
+            # Union with EmptyMarker is to make sure we get the benefit of marker
+            # simplifications.
+            dep.marker = MarkerUnion(*new_markers).union(EmptyMarker())
             by_constraint[constraint] = [dep]
 
         return [value[0] for value in by_constraint.values()]

src/poetry/puzzle/solver.py

@@ -181,8 +181,16 @@ def _solve(self) -> tuple[list[Package], list[int]]:
                             if _package.name == dep.name:
                                 continue
 
-                            if dep not in _package.requires:
+                            try:
+                                index = _package.requires.index(dep)
+                            except ValueError:
                                 _package.add_dependency(dep)
+                            else:
+                                _dep = _package.requires[index]
+                                if _dep.marker != dep.marker:
+                                    # marker of feature package is more accurate
+                                    # because it includes relevant extras
+                                    _dep.marker = dep.marker
             else:
                 final_packages.append(package)
                 depths.append(results[package])

tests/puzzle/test_solver.py

@@ -791,6 +791,45 @@ def test_solver_merge_extras_into_base_package_multiple_repos_fixes_5727(
     assert len(ops[0].package.requires) == 0, "a should not require itself"
 
 
+def test_solver_returns_extras_if_excluded_by_markers_without_extras(
+    solver: Solver, repo: Repository, package: ProjectPackage
+):
+    package.add_dependency(
+        Factory.create_dependency("A", {"version": "*", "extras": ["foo"]})
+    )
+
+    package_a = get_package("A", "1.0")
+    package_b = get_package("B", "1.0")
+
+    # mandatory dependency with marker
+    dep = get_dependency("B", "^1.0")
+    dep.marker = parse_marker("sys_platform != 'linux'")
+    package_a.add_dependency(dep)
+
+    # optional dependency with same constraint and no marker except for extra
+    dep = get_dependency("B", "^1.0", optional=True)
+    dep.marker = parse_marker("extra == 'foo'")
+    package_a.extras = {"foo": [dep]}
+    package_a.add_dependency(dep)
+
+    repo.add_package(package_a)
+    repo.add_package(package_b)
+
+    transaction = solver.solve()
+
+    ops = check_solver_result(
+        transaction,
+        [
+            {"job": "install", "package": package_b},
+            {"job": "install", "package": package_a},
+        ],
+    )
+    assert (
+        str(ops[1].package.requires[0].marker)
+        == 'sys_platform != "linux" or extra == "foo"'
+    )
+
+
 def test_solver_returns_prereleases_if_requested(
     solver: Solver, repo: Repository, package: ProjectPackage
 ):