Invalid Dynamic Dependency files are being produced from language C++ 20 or later programs

[MarcosASandoval]

On Windows

Description

Version of emscripten/emsdk:
Output of emcc -v

emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 4.0.15 (09f52557f0d48b65b8c724853ed8f4e8bf80e669)
clang version 22.0.0git (https:/github.com/llvm/llvm-project 3388d40684742e950b3c5d1d2dafe5a40695cfc1)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: C:\Users\user\emsdk\upstream\bin

Failing command line in full:
cmake --build .\out -v and emmake cmake --build .\out -v produce the same results

Output

Change Dir: 'D:/emscriptenDebug/out'

Run Build Command(s): C:/Users/user/AppData/Local/Microsoft/WinGet/Links/ninja.exe -v
[1/4] C:\WINDOWS\system32\cmd.exe /C ""C:/Users/user/emsdk/upstream/emscripten/emscan-deps" -format=p1689 -- C:\Users\user\emsdk\upstream\emscripten\em++.bat   -std=gnu++23 -x c++ D:/emscriptenDebug/demoApp.cpp -c -o CMakeFiles\demoApp.dir\demoApp.cpp.o -MT CMakeFiles\demoApp.dir\demoApp.cpp.o.ddi -MD -MF CMakeFiles\demoApp.dir\demoApp.cpp.o.ddi.d > CMakeFiles\demoApp.dir\demoApp.cpp.o.ddi.tmp && "C:/Program Files/CMake/bin/cmake.exe" -E rename CMakeFiles\demoApp.dir\demoApp.cpp.o.ddi.tmp CMakeFiles\demoApp.dir\demoApp.cpp.o.ddi"
[2/4] "C:\Program Files\CMake\bin\cmake.exe" -E cmake_ninja_dyndep --tdi=CMakeFiles\demoApp.dir\CXXDependInfo.json --lang=CXX --modmapfmt=clang --dd=CMakeFiles/demoApp.dir/CXX.dd @CMakeFiles/demoApp.dir/CXX.dd.rsp
FAILED: [code=1] CMakeFiles/demoApp.dir/CXX.dd CMakeFiles/demoApp.dir/CXXModules.json CMakeFiles/demoApp.dir/demoApp.cpp.o.modmap
"C:\Program Files\CMake\bin\cmake.exe" -E cmake_ninja_dyndep --tdi=CMakeFiles\demoApp.dir\CXXDependInfo.json --lang=CXX --modmapfmt=clang --dd=CMakeFiles/demoApp.dir/CXX.dd @CMakeFiles/demoApp.dir/CXX.dd.rsp
CMake Error: -E cmake_ninja_dyndep failed to parse CMakeFiles/demoApp.dir/demoApp.cpp.o.ddi* Line 1, Column 3
  Syntax error: value, object or array expected.

CMake Error: -E cmake_ninja_dyndep failed to parse ddi file CMakeFiles/demoApp.dir/demoApp.cpp.o.ddi
ninja: build stopped: subcommand failed.

Steps to reproduce

1. Run command emcmake cmake . -B out -G Ninja:
   Output

configure: cmake . -B out -G Ninja '-DCMAKE_TOOLCHAIN_FILE=C:\Users\user\emsdk\upstream\emscripten\cmake\Modules\Platform\Emscripten.cmake' -DCMAKE_CROSSCOMPILING_EMULATOR=C:/Users/user/emsdk/node/22.16.0_64bit/bin/node.exe
-- Configuring done (0.5s)
-- Generating done (0.0s)
-- Build files have been written to: D:/emscriptenDebug/out

2. Run build command cmake --build .\out -v or emmake cmake --build .\out -v
3. Receive above error

Minimal project setup

Root directory D:\emscriptenDebug\

Contents of file .\demoApp.cpp:

#include <iostream>

int main() {
    std::cout << "Hello World!" << std::endl;
}

Contents of file .\CMakeLists.txt:

cmake_minimum_required (VERSION 4.0)

project(
  emscriptenDebug
  VERSION 0.1
  DESCRIPTION "Minimal example project"
  LANGUAGES CXX
)

set(CMAKE_CXX_STANDARD 23)

add_executable(demoApp "demoApp.cpp")

Synopsis

Invalid DDI files are being produced in the output. There is a blank line, in this case's single .ddi file, followed by a line listing the output directory and a set of parentheses.

Removing this second line, D:\emscriptenDebug\out>(), and rerunning the build command produces a demoApp.wasm file and a demoApp.js file. Subsequent changes to demoApp.cpp will cause Ninja to rebuild and reproduce the invalid .ddi, causing the build to fail.

Invalid DDI file:

emscripten build generated file .\out\CMakeFiles\demoApp.dir\demoApp.cpp.o.ddi:

D:\emscriptenDebug\out>() 
{
  "revision": 0,
  "rules": [
    {
      "primary-output": "CMakeFiles\\demoApp.dir\\demoApp.cpp.o"
    }
  ],
  "version": 1
}

Valid DDI file

Running the configure, step 1 above, without the emcmake prefix, ie cmake . -B out -G Ninja followed by step 2 generates a properly formed DDI file. Builds succeed for this configuration.

Contents of CMake only generated file, .\out\CMakeFiles\demoApp.dir\demoApp.cpp.obj.ddi:

{
    "version": 1,
    "revision": 0,
    "rules": [
        {
            "primary-output": "CMakeFiles\\demoApp.dir\\demoApp.cpp.obj",
            "outputs": [
                "CMakeFiles\\demoApp.dir\\demoApp.cpp.obj.ddi"
            ],
            "requires": []
        }
    ]
}

Might also be worth mentioning that the output filenames differ slightly:

• .\out\CMakeFiles\demoApp.dir\demoApp.cpp.o.ddi for the emscripten configured build vs
• .\out\CMakeFiles\demoApp.dir\demoApp.cpp.obj.ddi CMake configured build