python-aliases: add warnAlias

pkgs/top-level/python-aliases.nix

@@ -27,6 +27,26 @@ let
     lib.mapAttrs (
       n: alias: removeDistribute (removeRecurseForDerivations (checkInPkgs n alias))
     ) aliases;
+
+  warnAlias =
+    msg: v:
+    if lib.isDerivation v then
+      lib.warnOnInstantiate msg v
+    else if lib.isAttrs v then
+      lib.mapAttrs (_: lib.warn msg) v
+    else if lib.isFunction v then
+      arg: lib.warn msg (v arg)
+    else if lib.isList v then
+      map (lib.warn msg) v
+    else
+      # Can’t do better than this, and a `throw` would be more
+      # disruptive for users…
+      #
+      # `nix search` flags up warnings already, so hopefully this won’t
+      # make things much worse until we have proper CI for aliases,
+      # especially since aliases of paths and numbers are presumably
+      # not common.
+      lib.warn msg v;
 in
 
 ### Deprecated aliases - for backward compatibility