CMake Error: ALIAS target "Boost::system" name collision when using vcpkg and FetchContent

[krmi]

Hi,

I am encountering a CMake configuration error when trying to include Crow via FetchContent (master branch) while managing my dependencies (specifically Boost) using vcpkg.

The Issue Vcpkg creates a Boost::system target via its toolchain file. However, Crow's CMakeLists.txt attempts to unconditionally create an alias with the exact same name, resulting in a fatal CMake error during configuration.

Steps to Reproduce

Use vcpkg.json to install Boost:

JSON

{
"name": "my-project",
"version-string": "0.1.0",
"dependencies": [ "boost-system", "boost-asio" ]
}
In CMakeLists.txt, find Boost via vcpkg and include Crow via FetchContent:

CMake

find_package(Boost REQUIRED CONFIG COMPONENTS system asio)

include(FetchContent)
FetchContent_Declare(
crow
GIT_REPOSITORY https://github.com/CrowCpp/Crow.git
GIT_TAG master
)
FetchContent_MakeAvailable(crow)
Error Output

Plaintext

CMake Error at .../vcpkg/scripts/buildsystems/vcpkg.cmake:653 (_add_library):
_add_library cannot create ALIAS target "Boost::system" because another
target with the same name already exists.
Call Stack (most recent call first):
build/_deps/crow-src/CMakeLists.txt:106 (add_library)
Proposed Solution In Crow's CMakeLists.txt (around line 106), the creation of the alias should be guarded to check if the target already exists.

Current code:

CMake

add_library(Boost::system ALIAS Boost::headers)
Suggested fix:

CMake

if(NOT TARGET Boost::system)
add_library(Boost::system ALIAS Boost::headers)
endif()
Environment

OS: Windows

Compiler: MSVC

Crow Version: Master

Dependency Manager: vcpkg (Manifest mode)

[gittiver]

Could you provide the fix as PR?

[schwitters]

workaround:
..
find_package(Boost CONFIG REQUIRED COMPONENTS system asio )
..
include(FetchContent)
set(CROW_USE_BOOST ON)
set(CROW_PATCH_SCRIPT "${CMAKE_BINARY_DIR}/patch_crow.cmake")
file(WRITE "${CROW_PATCH_SCRIPT}" "
message(STATUS "Patching Crow: Removing Boost conflict...")
set(TARGET_FILE "CMakeLists.txt")
if(EXISTS "${TARGET_FILE}")
file(READ "${TARGET_FILE}" file_content)
string(REPLACE
"add_library(Boost::system ALIAS Boost::headers)"
"# PATCHED BY ME: add_library(Boost::system ALIAS Boost::headers)"
file_content
"${file_content}"
)
file(WRITE "${TARGET_FILE}" "${file_content}")
endif()
")

FetchContent_Declare(
crow
GIT_REPOSITORY https://github.com/CrowCpp/Crow.git
GIT_TAG master
PATCH_COMMAND ${CMAKE_COMMAND} -P "${CROW_PATCH_SCRIPT}"
)
FetchContent_MakeAvailable(crow)