[CMake error] `FetchContent_MakeAvailable(volk)` breaks subsequent `FetchContent_MakeAvailable`

[xerpi]

Summary

I'm trying to use VOLK (as a static library) in my project, and for that I'm using FetchContent_MakeAvailable(volk).
After volk is made available, a subsequent FetchContent_MakeAvailable(...) (e.g. spdlog) fails with:

CMake Error at /usr/share/cmake/Modules/FetchContent.cmake:1591 (message):
  Internal error: SUBBUILD_DIR not set
Call Stack (most recent call first):
  /usr/share/cmake/Modules/FetchContent.cmake:2155:EVAL:2 (__FetchContent_doPopulation)
  /usr/share/cmake/Modules/FetchContent.cmake:2155 (cmake_language)
  /usr/share/cmake/Modules/FetchContent.cmake:2394 (__FetchContent_Populate)
  CMakeLists.txt:26 (FetchContent_MakeAvailable)

This appears to be caused by VOLK mutating global CMake directory variables.

Environment

• CMake: 4.2.3
• OS: Linux (x86_64)
• VOLK: main (511fff053e190316919733fb148b70946f139a9e)

Minimal Reproducer

CMakeLists.txt:

cmake_minimum_required(VERSION 3.24)
project(volk_fetchcontent_repro LANGUAGES CXX)

set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

include(FetchContent)

# Avoid unrelated cpu_features min-CMake issue on this setup
set(VOLK_CPU_FEATURES OFF CACHE BOOL "Disable cpu_features for repro" FORCE)

# 1) Fetch VOLK first
FetchContent_Declare(
  volk
  GIT_REPOSITORY https://github.com/gnuradio/volk.git
  GIT_TAG main
)
FetchContent_MakeAvailable(volk)

# 2) Then fetch another project (spdlog)
FetchContent_Declare(
  spdlog
  GIT_REPOSITORY https://github.com/gabime/spdlog.git
  GIT_TAG v1.17.0
)
FetchContent_MakeAvailable(spdlog)

add_executable(repro main.cpp)
target_link_libraries(repro PRIVATE volk spdlog::spdlog)

main.cpp:

#include <volk/volk.h>
#include <spdlog/spdlog.h>

int main()
{
    spdlog::info("Includes from VOLK and spdlog are visible");
    return 0;
}

Repro Command

cmake --fresh -S . -B build

Actual Result

Configuration fails at the second FetchContent_MakeAvailable(spdlog) with Internal error: SUBBUILD_DIR not set.

Expected Result

FetchContent_MakeAvailable(spdlog) should work normally after FetchContent_MakeAvailable(volk).

Additional Notes

• spdlog alone configures successfully.
• spdlog first, then volk, also configures successfully.
• So this appears order-dependent (volk first triggers it).

Suspected Root Cause

VOLK top-level CMakeLists.txt reassigns global CMake vars:

set(CMAKE_SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR}) #allows this to be a sub-project
set(CMAKE_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR}) #allows this to be a sub-project

These should not be overridden by a subproject and likely break FetchContent internals for later dependencies.

Suggested Fix

Remove those assignments and use CMAKE_CURRENT_SOURCE_DIR / CMAKE_CURRENT_BINARY_DIR directly where needed.

[xerpi]

Additional root cause identified for the SUBBUILD_DIR not set failure:

VOLK still had a legacy compatibility module (cmake/Modules/CMakeParseArgumentsCopy.cmake) and included it in module code.
That module defines CMAKE_PARSE_ARGUMENTS(...), which can shadow the built-in cmake_parse_arguments(...) (CMake command names are case-insensitive).

FetchContent.cmake relies on the built-in parser with PARSE_ARGV, so this shadowing can break argument parsing in __FetchContent_doPopulation(), leaving ARG_SUBBUILD_DIR unset and causing:

Internal error: SUBBUILD_DIR not set

I fixed (#816) this by:

• removing include(CMakeParseArgumentsCopy) usage in VOLK modules
• deleting cmake/Modules/CMakeParseArgumentsCopy.cmake

Related prior work in GNU Radio:

• retire CMakeParseArgumentsCopy gnuradio#7716
• cmake: Remove CMakeParseArgumentsCopy.cmake gnuradio#7996