explain requires-python ignored links

src/pip/_internal/index/package_finder.py

@@ -225,7 +225,10 @@ class LinkEvaluator:
         if not supports_python:
             # Return None for the reason text to suppress calling
             # _log_skipped_link().
-            return (False, None)
+            return (
+                False,
+                '{} requires-python {}'.format(version, link.requires_python),
+            )
 
         logger.debug("Found link %s, version: %s", link, version)
 
@@ -609,7 +612,8 @@ class PackageFinder:
         self.format_control = format_control
 
         # These are boring links that have already been logged somehow.
-        self._logged_links: Set[Link] = set()
+        self._logged_links = set()  # type: Set[Link]
+        self._logged_links_rp = set()  # type: Set[str]
 
     # Don't include an allow_yanked default value to make sure each call
     # site considers whether yanked releases are allowed. This also causes
@@ -699,6 +703,12 @@ class PackageFinder:
             ignore_requires_python=self._ignore_requires_python,
         )
 
+    def logged_links_rp(self):
+        # type: () -> List[str]
+        skips = [skip for skip in self._logged_links_rp]
+        skips.sort()
+        return skips
+
     def _sort_links(self, links: Iterable[Link]) -> List[Link]:
         """
         Returns elements of links in order, non-egg links first, egg links
@@ -731,7 +741,9 @@ class PackageFinder:
         """
         is_candidate, result = link_evaluator.evaluate_link(link)
         if not is_candidate:
-            if result:
+            if 'requires-python' in result:
+                self._log_skipped_link_rp(result)
+            else:
                 self._log_skipped_link(link, reason=result)
             return None
 
@@ -741,6 +753,11 @@ class PackageFinder:
             version=result,
         )
 
+    def _log_skipped_link_rp(self, reason):
+        # type: (str) -> None
+        if reason not in self._logged_links_rp:
+            self._logged_links_rp.add(reason)
+
     def evaluate_links(
         self, link_evaluator: LinkEvaluator, links: Iterable[Link]
     ) -> List[InstallationCandidate]:

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

@@ -579,6 +579,7 @@ class Factory:
             req_disp = f"{req} (from {parent.name})"
 
         cands = self._finder.find_all_candidates(req.project_name)
+        skips = self._finder.logged_links_rp()
         versions = [str(v) for v in sorted({c.version for c in cands})]
 
         logger.critical(
@@ -594,6 +595,12 @@ class Factory:
                 "using the '-r' flag to install the packages listed in "
                 "requirements.txt"
             )
+        logger.critical(
+            "Found versions that do not satisfy the requirement %s "
+            "(from versions: %s)",
+            req_disp,
+            ", ".join(skips) or "none",
+        )
 
         return DistributionNotFound(f"No matching distribution found for {req}")