# CMake falsely executes commands inside of if

**URL:** https://discourse.cmake.org/t/cmake-falsely-executes-commands-inside-of-if/3370
**Category:** Development
**Created:** [May 19, 2021, 8:30pm UTC](https://discourse.cmake.org/t/cmake-falsely-executes-commands-inside-of-if/3370 "2021-05-19T20:30:43Z")
**Posts on this page:** 4
**Page:** 1

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### Author: ![Clemens](https://discourse.cmake.org/user_avatar/discourse.cmake.org/clemens/32/1461_2.png) [@Clemens](https://discourse.cmake.org/u/Clemens)
#### Post date: [May 19, 2021, 8:30pm UTC](https://discourse.cmake.org/t/cmake-falsely-executes-commands-inside-of-if/3370/1 "2021-05-19T20:30:43Z")

</div>

# Issue

I wanted to build LLVM from source with LibXml2 on Windows.  
The easiest option for LibXml2 was to use vcpkg.  
I am using a pull request (see Reproduction), which integrates LibXml2 upstream CMake support.  
The problem that I am currently facing is that configuring of LLVM fails with the following error:

```auto
CMake Error at E:/git/vcpkg/installed/x64-windows-static/share/libxml2/vcpkg-cmake-wrapper.cmake:41 (target_link_libraries):
  Cannot specify link libraries for target "LibXml2::LibXml2" which is not
  built by this project.
Call Stack (most recent call first):
  E:/git/vcpkg/scripts/buildsystems/vcpkg.cmake:815 (include)
  E:/git/llvm-project/clang/CMakeLists.txt:207 (find_package)

-- Configuring incomplete, errors occurred!
See also "E:/git/llvm-project/build/CMakeFiles/CMakeOutput.log".
See also "E:/git/llvm-project/build/CMakeFiles/CMakeError.log".

```

The problem indicates that detecting LibXml2 by Clang (a submodule of LLVM) fails. The weird thing is that LibXml2 was correctly detected by the LLVM CMakeLists.

The code that failes looks like this:

```auto
if(TARGET LibXml2::LibXml2)
   target_link_libraries(LibXml2::LibXml2 INTERFACE ${ICONV_LIBRARIES})

```

This tells me that there is probably a bug in CMake, because CMake executes `target_link_libraries` although the target `LibXml2::LibXml2` doesn’t exist.  
I build CMake from source and wanted to debug this situation, but I didn’t got really far. I only found out that for the if command from above the call to `cmMakefile::ExecuteCommand` directly returns, because `IsFunctionBlocked` returns true. And then in `cmTargetLinkLibrariesCommand` the second statement from above produces the errormessage from above, because it cant find the target, **although the if condition above was OK**.

# Reproduction

- Checkout this [branch](https://github.com/Neumann-A/vcpkg/tree/update_libxml2) somewhere
- Inside run `.\bootstrap-vcpkg.bat` and `.\vcpkg.exe install libxml2:x64-windows-static`
- Checkout [llvm-project](https://github.com/llvm/llvm-project) somewhere
- Run `cmake -B build -DVCPKG_TARGET_TRIPLET=x64-windows-static -DCMAKE_TOOLCHAIN_FILE="<VCPKG_ROOT>/scripts/buildsystems/vcpkg.cmake" -DLLVM_ENABLE_PROJECTS=clang -DLLVM_ENABLE_LIBXML2=FORCE_ON llvm-project/llvm` **replace \<VCPKG\_ROOT\> with the location, where you cloned the vcpkg branch from above**

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### Author: ![ben.boeckel](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/b/ea5d25/32.png) [@ben.boeckel](https://discourse.cmake.org/u/ben.boeckel)
#### Post date: [May 19, 2021, 11:55pm UTC](https://discourse.cmake.org/t/cmake-falsely-executes-commands-inside-of-if/3370/2 "2021-05-19T23:55:05Z")

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I assume that you’re using `find_package(LibXml2)`. This does create a `LibXml2::LibXml2` target. It’s `IMPORTED`, but it exists. That conditional should be fixed to either only run when the vendored libxml2 was built or, if that is difficult, check if it is an `IMPORTED` target or not.

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<div class="post-metadata">

### Author: ![Clemens](https://discourse.cmake.org/user_avatar/discourse.cmake.org/clemens/32/1461_2.png) [@Clemens](https://discourse.cmake.org/u/Clemens)
#### Post date: [May 20, 2021, 11:21am UTC](https://discourse.cmake.org/t/cmake-falsely-executes-commands-inside-of-if/3370/3 "2021-05-20T11:21:53Z")

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The `LibXml2::LibXml2` is always an `IMPORTED` Target. This doesn’t seem to be a problem, since I don’t get this error when configuring LLVM (which also calls this script and code). This only errors when I also configure clang.

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<div class="post-metadata">

### Author: ![ben.boeckel](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/b/ea5d25/32.png) [@ben.boeckel](https://discourse.cmake.org/u/ben.boeckel)
#### Post date: [May 20, 2021, 12:47pm UTC](https://discourse.cmake.org/t/cmake-falsely-executes-commands-inside-of-if/3370/4 "2021-05-20T12:47:57Z")

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This really sounds like a problem in the `clang` codebase, not in CMake itself. Just because a target exists does not mean that `target_link_libraries` on it will succeed (because it may be `IMPORTED` or even `add_custom_target`).