Merge commit 'b09661d41fb93562fd53f31574dbf781b130ac44'

3 files changed, 14 insertions, 2 deletions

diff --git a/nixpkgs/lib/default.nix b/nixpkgs/lib/default.nix
index ccfee2ebe303..8e29ef5c4201 100644
--- a/nixpkgs/lib/default.nix
+++ b/nixpkgs/lib/default.nix
@@ -116,7 +116,7 @@ let
       pushDownProperties dischargeProperties filterOverrides
       sortProperties fixupOptionType mkIf mkAssert mkMerge mkOverride
       mkOptionDefault mkDefault mkForce mkVMOverride
-      mkOrder mkBefore mkAfter mkAliasDefinitions
+      mkFixStrictness mkOrder mkBefore mkAfter mkAliasDefinitions
       mkAliasAndWrapDefinitions fixMergeModules mkRemovedOptionModule
       mkRenamedOptionModule mkMergedOptionModule mkChangedOptionModule
       mkAliasOptionModule doRename;
diff --git a/nixpkgs/lib/modules.nix b/nixpkgs/lib/modules.nix
index 58c6cda58e49..ab2bc4f7f8e2 100644
--- a/nixpkgs/lib/modules.nix
+++ b/nixpkgs/lib/modules.nix
@@ -713,6 +713,8 @@ rec {
   mkForce = mkOverride 50;
   mkVMOverride = mkOverride 10; # used by ‘nixos-rebuild build-vm’
 
+  mkFixStrictness = lib.warn "lib.mkFixStrictness has no effect and will be removed. It returns its argument unmodified, so you can just remove any calls." id;
+
   mkOrder = priority: content:
     { _type = "order";
       inherit priority content;
diff --git a/nixpkgs/lib/types.nix b/nixpkgs/lib/types.nix
index f47a1f92de72..c35f055e17f5 100644
--- a/nixpkgs/lib/types.nix
+++ b/nixpkgs/lib/types.nix
@@ -581,7 +581,17 @@ rec {
       in
       mkOptionType rec {
         name = "enum";
-        description = "one of ${concatMapStringsSep ", " show values}";
+        description =
+          # Length 0 or 1 enums may occur in a design pattern with type merging
+          # where an "interface" module declares an empty enum and other modules
+          # provide implementations, each extending the enum with their own
+          # identifier.
+          if values == [] then
+            "impossible (empty enum)"
+          else if builtins.length values == 1 then
+            "value ${show (builtins.head values)} (singular enum)"
+          else
+            "one of ${concatMapStringsSep ", " show values}";
         check = flip elem values;
         merge = mergeEqualOption;
         functor = (defaultFunctor name) // { payload = values; binOp = a: b: unique (a ++ b); };