Add hint when resolution impossible causes have no versions available

news/13588.feature.rst

@@ -0,0 +1 @@
+On ``ResolutionImpossible`` errors, include a note about causes with no candidates.

src/pip/_internal/resolution/resolvelib/factory.py

@@ -711,6 +711,21 @@ def _report_single_requirement_conflict(
 
         return DistributionNotFound(f"No matching distribution found for {req}")
 
+    def _has_any_candidates(self, project_name: str) -> bool:
+        """
+        Check if there are any candidates available for the project name.
+        """
+        return any(
+            self.find_candidates(
+                project_name,
+                requirements={project_name: []},
+                incompatibilities={},
+                constraint=Constraint.empty(),
+                prefers_installed=True,
+                is_satisfied_by=lambda r, c: True,
+            )
+        )
+
     def get_installation_error(
         self,
         e: ResolutionImpossible[Requirement, Candidate],
@@ -796,6 +811,22 @@ def describe_trigger(parent: Candidate) -> str:
             spec = constraints[key].specifier
             msg += f"\n    The user requested (constraint) {key}{spec}"
 
+        # Check for causes that had no candidates
+        causes = set()
+        for req, _ in e.causes:
+            causes.add(req.name)
+
+        no_candidates = {c for c in causes if not self._has_any_candidates(c)}
+        if no_candidates:
+            msg = (
+                msg
+                + "\n\n"
+                + "Additionally, some packages in these conflicts have no "
+                + "matching distributions available for your environment:"
+                + "\n    "
+                + "\n    ".join(sorted(no_candidates))
+            )
+
         msg = (
             msg
             + "\n\n"

tests/functional/test_new_resolver_errors.py

@@ -6,6 +6,7 @@
     create_basic_wheel_for_package,
     create_test_package_with_setup,
 )
+from tests.lib.wheel import make_wheel
 
 
 def test_new_resolver_conflict_requirements_file(
@@ -132,3 +133,61 @@ def test_new_resolver_checks_requires_python_before_dependencies(
     # Setuptools produces wheels with normalized names.
     assert "pkg_dep" not in result.stderr, str(result)
     assert "pkg_dep" not in result.stdout, str(result)
+
+
+def test_new_resolver_no_versions_available_hint(script: PipTestEnvironment) -> None:
+    """
+    Test hint that no package candidate is available at all,
+    when ResolutionImpossible occurs.
+    """
+    wheel_house = script.scratch_path.joinpath("wheelhouse")
+    wheel_house.mkdir()
+
+    incompatible_dep_wheel = make_wheel(
+        name="incompatible-dep",
+        version="1.0.0",
+        wheel_metadata_updates={"Tag": ["py3-none-fakeplat"]},
+    )
+    incompatible_dep_wheel.save_to(
+        wheel_house.joinpath("incompatible_dep-1.0.0-py3-none-fakeplat.whl")
+    )
+
+    # Create multiple versions of a package that depend on the incompatible dependency
+    requesting_pkg_v1 = make_wheel(
+        name="requesting-pkg",
+        version="1.0.0",
+        metadata_updates={"Requires-Dist": ["incompatible-dep==1.0.0"]},
+    )
+    requesting_pkg_v1.save_to(
+        wheel_house.joinpath("requesting_pkg-1.0.0-py2.py3-none-any.whl")
+    )
+
+    requesting_pkg_v2 = make_wheel(
+        name="requesting-pkg",
+        version="2.0.0",
+        metadata_updates={"Requires-Dist": ["incompatible-dep==1.0.0"]},
+    )
+    requesting_pkg_v2.save_to(
+        wheel_house.joinpath("requesting_pkg-2.0.0-py2.py3-none-any.whl")
+    )
+
+    # Attempt to install the requesting package
+    result = script.pip(
+        "install",
+        "--no-cache-dir",
+        "--no-index",
+        "--find-links",
+        str(wheel_house),
+        "requesting-pkg",
+        expect_error=True,
+    )
+
+    # Check that ResolutionImpossible error occurred
+    assert "ResolutionImpossible" in result.stderr, str(result)
+
+    # Check that the new hint message is present
+    assert (
+        "Additionally, some packages in these conflicts have no "
+        "matching distributions available for your environment:\n"
+        "    incompatible-dep\n" in result.stdout
+    ), str(result)