flake/templates: propagate the template's checks to the main flake's

flake-modules/templates.nix

@@ -1,8 +1,60 @@
 {
+  self,
+  inputs,
+  ...
+}: {
   flake.templates = {
     default = {
       path = ../templates/simple;
       description = "A simple nix flake template for getting started with nixvim";
     };
   };
+
+  # The following adds the template flake's checks to the main (current) flake's checks.
+  # It ensures that the template's own checks are successful.
+  perSystem = {
+    pkgs,
+    system,
+    lib,
+    ...
+  }: {
+    checks = let
+      # Approximates https://github.com/NixOS/nix/blob/7cd08ae379746749506f2e33c3baeb49b58299b8/src/libexpr/flake/call-flake.nix#L46
+      # s/flake.outputs/args.outputs/
+      callFlake = args @ {
+        inputs,
+        outputs,
+        sourceInfo,
+      }: let
+        outputs = args.outputs (inputs // {self = result;});
+        result =
+          outputs
+          // sourceInfo
+          // {
+            inherit inputs outputs sourceInfo;
+            _type = "flake";
+          };
+      in
+        result;
+
+      templateFlakeOuputs = callFlake {
+        inputs = {
+          inherit (inputs) flake-parts nixpkgs;
+          nixvim = self;
+        };
+        # Import and read the `outputs` field of the template flake.
+        inherit (import ../templates/simple/flake.nix) outputs;
+        sourceInfo = {};
+      };
+
+      templateChecks = templateFlakeOuputs.checks.${system};
+    in
+      lib.concatMapAttrs
+      (
+        checkName: check: {
+          "template-${checkName}" = check;
+        }
+      )
+      templateChecks;
+  };
 }