provider: allow fallback to installed packages

src/poetry/puzzle/provider.py

@@ -35,6 +35,7 @@ from poetry.mixology.term import Term
 from poetry.packages import DependencyPackage
 from poetry.packages.package_collection import PackageCollection
 from poetry.puzzle.exceptions import OverrideNeeded
+from poetry.repositories.exceptions import PackageNotFound
 from poetry.utils.helpers import download_file
 from poetry.vcs.git import Git
 
@@ -46,10 +47,12 @@ if TYPE_CHECKING:
 
     from poetry.core.packages.dependency import Dependency
     from poetry.core.packages.package import Package
+    from poetry.core.packages.specification import PackageSpecification
     from poetry.core.semver.version_constraint import VersionConstraint
     from poetry.core.version.markers import BaseMarker
 
     from poetry.repositories import Pool
+    from poetry.repositories import Repository
     from poetry.utils.env import Env
 
 
@@ -124,6 +127,7 @@ class Provider:
         pool: Pool,
         io: Any,
         env: Env | None = None,
+        installed: Repository | None = None,
     ) -> None:
         self._package = package
         self._pool = pool
@@ -136,6 +140,7 @@ class Provider:
         self._deferred_cache: dict[Dependency, Package] = {}
         self._load_deferred = True
         self._source_root: Path | None = None
+        self._installed = installed
 
     @property
     def pool(self) -> Pool:
@@ -185,6 +190,36 @@ class Provider:
                 f" package's name: {package.name}"
             )
 
+    def search_for_installed_packages(
+        self,
+        specification: PackageSpecification,
+    ) -> list[Package]:
+        """
+        Search for installed packages, when available, that provides the given
+        specification.
+
+        This is useful when dealing with packages that are under development, not
+        published on package sources and/or only available via system installations.
+        """
+        if not self._installed:
+            return []
+
+        logger.debug(
+            "Falling back to installed packages to discover metadata for <c2>%s</>",
+            specification.complete_name,
+        )
+        packages = [
+            package
+            for package in self._installed.packages
+            if package.provides(specification)
+        ]
+        logger.debug(
+            "Found <c2>%d</> compatible packages for <c2>%s</>",
+            len(packages),
+            specification.complete_name,
+        )
+        return packages
+
     def search_for(
         self,
         dependency: (
@@ -227,6 +262,9 @@ class Provider:
                 reverse=True,
             )
 
+        if not packages:
+            packages = self.search_for_installed_packages(dependency)
+
         return PackageCollection(dependency, packages)
 
     def search_for_vcs(self, dependency: VCSDependency) -> list[Package]:
@@ -478,6 +516,7 @@ class Provider:
             "url",
             "git",
         }:
+            try:
                 package = DependencyPackage(
                     package.dependency,
                     self._pool.package(
@@ -487,6 +526,19 @@ class Provider:
                         repository=package.dependency.source_name,
                     ),
                 )
+            except PackageNotFound as e:
+                try:
+                    package = next(
+                        DependencyPackage(
+                            package.dependency,
+                            pkg,
+                        )
+                        for pkg in self.search_for_installed_packages(
+                            package.dependency
+                        )
+                    )
+                except StopIteration:
+                    raise e from e
             requires = package.requires
         else:
             requires = package.requires

src/poetry/puzzle/solver.py

@@ -58,7 +58,9 @@ class Solver:
         self._io = io
 
         if provider is None:
-            provider = Provider(self._package, self._pool, self._io)
+            provider = Provider(
+                self._package, self._pool, self._io, installed=installed
+            )
 
         self._provider = provider
         self._overrides: list[dict[DependencyPackage, dict[str, Dependency]]] = []

tests/puzzle/test_solver.py

@@ -87,7 +87,12 @@ def solver(
     io: NullIO,
 ) -> Solver:
     return Solver(
-        package, pool, installed, locked, io, provider=Provider(package, pool, io)
+        package,
+        pool,
+        installed,
+        locked,
+        io,
+        provider=Provider(package, pool, io, installed=installed),
     )
 
 
@@ -174,6 +179,36 @@ def test_install_non_existing_package_fail(
         solver.solve()
 
 
+def test_install_unpublished_package_does_not_fail(
+    installed: InstalledRepository,
+    solver: Solver,
+    repo: Repository,
+    package: ProjectPackage,
+):
+    package.add_dependency(Factory.create_dependency("B", "1"))
+
+    package_a = get_package("A", "1.0")
+    package_b = get_package("B", "1")
+    package_b.add_dependency(Factory.create_dependency("A", "1.0"))
+
+    repo.add_package(package_a)
+    installed.add_package(package_b)
+
+    transaction = solver.solve()
+
+    check_solver_result(
+        transaction,
+        [
+            {"job": "install", "package": package_a},
+            {
+                "job": "install",
+                "package": package_b,
+                "skipped": True,  # already installed
+            },
+        ],
+    )
+
+
 def test_solver_with_deps(solver: Solver, repo: Repository, package: ProjectPackage):
     package.add_dependency(Factory.create_dependency("A", "*"))