Minimal example.

CMakeLists.txt

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+cmake_minimum_required(VERSION 3.8)
+project(test_project)
+
+set(CMAKE_POSITION_INDEPENDENT_CODE ON)
+
+set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
+
+set(CMAKE_CXX_STANDARD 20)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_CXX_SCAN_FOR_MODULES ON)
+
+add_executable(modules_test)
+target_sources(modules_test
+        PRIVATE main.cpp
+        PUBLIC FILE_SET CXX_MODULES FILES module.cppm
+)
+

flake.lock

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+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1725930920,
+        "narHash": "sha256-RVhD9hnlTT2nJzPHlAqrWqCkA7T6CYrP41IoVRkciZM=",
+        "owner": "nixos",
+        "repo": "nixpkgs",
+        "rev": "44a71ff39c182edaf25a7ace5c9454e7cba2c658",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nixos",
+        "ref": "nixos-24.05",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

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+{
+
+  nixConfig.bash-prompt = "[shell $(pwd)]$ ";
+
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-24.05";
+  };
+
+  outputs = { self, nixpkgs, ... }:
+    let
+      system = "x86_64-linux";
+      pkgs = import nixpkgs { inherit system; };
+    in
+    {
+
+      formatter.x86_64-linux = nixpkgs.legacyPackages.x86_64-linux.nixpkgs-fmt;
+
+      packages.x86_64-linux.default = pkgs.llvmPackages_17.libcxxStdenv.mkDerivation {
+        name = "modules_test";
+        src = ./.;
+        nativeBuildInputs = with pkgs; [
+          cmake
+          llvmPackages_17.clang-tools
+          ninja
+        ];
+        buildInputs = with pkgs; [
+          llvmPackages_17.clang-tools
+        ];
+        configurePhase = ''
+          cmake . -GNinja
+        '';
+
+        buildPhase = ''
+          ninja modules_test
+        '';
+
+        installPhase = ''
+          ls
+          mkdir -p $out/bin
+          cp modules_test $out/bin
+        '';
+      };
+    };
+}

main.cpp

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+// Copyright (c) 2024 University College Roosevelt
+//
+// All rights reserved.
+
+//
+// Created by werner on 9/11/24.
+//
+
+import Foomod;
+
+int main() {
+	return add(1,2);
+}

module.cppm

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+// Copyright (c) 2024 University College Roosevelt
+//
+// All rights reserved.
+
+//
+// Created by werner on 9/11/24.
+//
+
+module;
+
+#include <iostream>
+
+export module Foomod;
+
+export int add(int a, int b) {
+
+	std::cout << "Look at me using iostream!" << std::endl;
+
+	return a + b;
+}