Add hint when resolution impossible causes have no versions

Author: notatallshaw

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

@@ -711,6 +711,13 @@ 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.
+        """
+        candidates = self._finder.find_all_candidates(project_name)
+        return any(not c.link.is_yanked for c in candidates)
+
     def get_installation_error(
         self,
         e: ResolutionImpossible[Requirement, Candidate],
@@ -796,6 +803,22 @@ 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"
+                + "Some conflict cause(s) have no "
+                + "available versions for your environment:"
+                + "\n "
+                + "\n ".join(sorted(no_candidates))
+            )
+
         msg = (
             msg
             + "\n\n"