provider: consider explicit source when searching for a locked packag…

…e with a source reference in the repository pool

src/poetry/puzzle/provider.py

@@ -491,7 +491,9 @@ def complete_package(
                         package.pretty_name,
                         package.version,
                         extras=list(dependency.extras),
-                        repository_name=dependency.source_name,
+                        repository_name=(
+                            dependency.source_name or package.source_reference
+                        ),
                     ),
                 )
             except PackageNotFound as e:

tests/puzzle/test_provider.py

@@ -22,6 +22,7 @@
 from poetry.puzzle.provider import IncompatibleConstraintsError
 from poetry.puzzle.provider import Provider
 from poetry.repositories.repository import Repository
+from poetry.repositories.repository_pool import Priority
 from poetry.repositories.repository_pool import RepositoryPool
 from poetry.utils.env import EnvCommandError
 from poetry.utils.env import MockEnv as BaseMockEnv
@@ -783,6 +784,24 @@ def test_complete_package_fetches_optional_vcs_dependency_only_if_requested(
         spy.assert_not_called()
 
 
+def test_complete_package_finds_locked_package_in_explicit_source(
+    pool: RepositoryPool, provider: Provider
+) -> None:
+    package = Package("a", "1.0", source_reference="explicit")
+    explicit_repo = Repository("explicit")
+    explicit_repo.add_package(package)
+    pool.add_repository(explicit_repo, priority=Priority.EXPLICIT)
+
+    dependency = package.to_dependency()
+    # complete_package() must succeed even if the dependency does not have an explicit
+    # source. This can be the case if the dependency is a transitive dependency and
+    # there is a direct dependency with an explicit source.
+    dependency.source_name = None
+
+    # must not fail
+    provider.complete_package(DependencyPackage(dependency, package))
+
+
 def test_source_dependency_is_satisfied_by_direct_origin(
     provider: Provider, repository: Repository
 ) -> None:

tests/puzzle/test_solver.py

@@ -3826,7 +3826,7 @@ def test_solver_should_not_update_same_version_packages_if_installed_has_no_sour
         "1.0.0",
         source_type="legacy",
         source_url="https://foo.bar",
-        source_reference="custom",
+        source_reference="repo",
     )
     repo.add_package(foo)