[clang] [windows] [modules] undefined symbol when exporting variable from shared library

[Silverlan]

Clang version: 21.1.8 (Downloaded from this repository)
Also tested on trunk (at commit faf140a), which produces the same results.

Full source code for the minimal reproduction code, including GitHub workflows:
https://github.com/Silverlan/tmp_clang_errors/tree/undefined-symbol

Full build log: https://github.com/Silverlan/tmp_clang_errors/actions/runs/20599656929/job/59161977160

Error Message:

 lld-link: error: undefined symbol: struct TestVector const test::cexpr_test_vector
>>> referenced by CMakeFiles/test_program.dir/main.cpp.obj:(main)
clang++: error: linker command failed with exit code 1 (use -v to see invocation)

Description

This bug happens when trying to access a variable exported from a c++20 module from a dynamic library when using clang (I have not tested clang-cl) on Windows. On Linux it compiles and links without any issues.

I've narrowed the problem down as much as I can. The reproduction project consists of a main application ("test_program"), as well as a dynamically linked library ("test_library"), which exports a c++20 module ("test_module"). Here is a short description of the code:

Code

test_library
This is a shared library, which exports a c++20 module with some simple objects.

export module test_module;

#ifdef _WIN32
#define API __declspec(dllexport)
#else
#define API __attribute__((visibility("default")))
#endif

export {
    struct API TestVector {
        float x;
        float y;
        float z;
    };
}

export namespace test {
    // These are causing "undefined symbol" errors in certain circumstances (see test cases in main program)
    constexpr TestVector cexpr_test_vector = TestVector{1.f,2.f,3.f};
    API TestVector dllexp_test_vector = TestVector{1.f,2.f,3.f};
    //

    API TestVector dllexp_get_test_vector() {return TestVector{1.f,2.f,3.f};}
    API void test_function(const TestVector &v)
    {
    }
}

test_program
The program links against test_library and contains several test cases which can be switched between when configuring the project with CMake (using the -DTEST_CASE=X option). Some cases trigger the linking error, others do not (see comments below for each test case).

#include <iostream>

import test_module;

// Test Case 0: Access member of constexpr vector from module test_module
// Result: Build successful
#if TEST_CASE == 0

int main() {
    std::cout<<test::cexpr_test_vector.x<<std::endl;
    return 0;
}

#endif

// Test Case 1: Call test_function from module test_module with constexpr vector
// Result: lld-link: error: undefined symbol: struct TestVector const test::cexpr_test_vector
#if TEST_CASE == 1

int main() {
    test::test_function(test::cexpr_test_vector);
    return 0;
}

#endif

// Test Case 2: Same as Test Case 1, but with locally created vector
// Result: Build successful
#if TEST_CASE == 2

int main() {
    test::test_function(TestVector {1.f, 2.f, 3.4});
    return 0;
}

#endif

// Test Case 3: Calling a locally defined function with constexpr vector as argument
// Result: Build successful
#if TEST_CASE == 3

static void print_vec(const TestVector v)
{
    std::cout<<v.x<<","<<v.y<<","<<v.z<<std::endl;
}

int main() {
    print_vec(test::cexpr_test_vector);
    return 0;
}

#endif

// Test Case 4: Same as Test Case 3, but with optimization turned off
// Result: lld-link: error: undefined symbol: struct TestVector const test::cexpr_test_vector
#if TEST_CASE == 4

#pragma clang optimize off
static void print_vec(const TestVector v)
{
    std::cout<<v.x<<","<<v.y<<","<<v.z<<std::endl;
}

int main() {
    print_vec(test::cexpr_test_vector);
    return 0;
}
#pragma clang optimize on

#endif

// Test Case 5: Same as Test Case 0, but using dllexported vector instead of constexpr
// Result: lld-link: error: undefined symbol: struct TestVector test::dllexp_test_vector
#if TEST_CASE == 5

int main() {
    std::cout<<test::dllexp_test_vector.x<<std::endl;
    return 0;
}

#endif

// Test Case 6: Same as Test Case 0, but using dllexported getter-function for vector instead of constexpr
// Result: Build successful
#if TEST_CASE == 6

int main() {
    std::cout<<test::dllexp_get_test_vector().x<<std::endl;
    return 0;
}

#endif

All test cases build and link successfully with clang on Linux.
Interestingly, when building with MSVC on Windows, Case 0 and 1 also fail with a similar error as the one clang reports:

main.cpp.obj : error LNK2019: unresolved external symbol "struct TestVector const test::cexpr_test_vector" (?cexpr_test_vector@test@@3UTestVector@@B::<!test_module>) referenced in function main

Build instructions:

Run from visual studio 2022 command prompt (replace clang executable paths CMAKE_C_COMPILER and CMAKE_CXX_COMPILER):

git clone --single-branch --branch "undefined-symbol" https://github.com/Silverlan/tmp_clang_errors.git
cd tmp_clang_errors
set root=%cd%
cd test_program
mkdir build
cd build
cmake -S .. -B . -G "Ninja" -DCMAKE_C_COMPILER="%root%/clang/bin/clang.exe"  -DCMAKE_CXX_COMPILER="%root%/clang/bin/clang++.exe" -DCMAKE_INSTALL_PREFIX:PATH="%root%/install" -DCMAKE_CXX_SCAN_FOR_MODULES=1 -DCMAKE_EXPORT_COMPILE_COMMANDS=1 -DCMAKE_BUILD_TYPE=Release -DCMAKE_POLICY_VERSION_MINIMUM=3.5 -DTEST_CASE=0
cmake --build .
cmake --install .
%root%/install/test_program.exe

(Change -DTEST_CASE=0 for each test case.)
(Alternatively you can fork the undefined-symbol branch of the test project repository and run the GitHub workflow.)

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: Silverlan (Silverlan)

Details

Clang version: 21.1.8 (Downloaded from this repository)

Full source code for the minimal reproduction code, including GitHub workflows:
https://github.com/Silverlan/tmp_clang_errors/tree/undefined-symbol

Full build log: https://github.com/Silverlan/tmp_clang_errors/actions/runs/20599656929/job/59161977160

Error Message:

 lld-link: error: undefined symbol: struct TestVector const test::cexpr_test_vector
>>> referenced by CMakeFiles/test_program.dir/main.cpp.obj:(main)
clang++: error: linker command failed with exit code 1 (use -v to see invocation)

Description

This bug happens when trying to access a variable exported from a c++20 module from a dynamic library when using clang (I have not tested clang-cl) on Windows. On Linux it compiles and links without any issues.

I've narrowed the problem down as much as I can. The reproduction project consists of a main application ("test_program"), as well as a dynamically linked library ("test_library"), which exports a c++20 module ("test_module"). Here is a short description of the code:

Code

test_library
This is a shared library, which exports a c++20 module with some simple objects.

export module test_module;

#ifdef _WIN32
#define API __declspec(dllexport)
#else
#define API __attribute__((visibility("default")))
#endif

export {
    struct API TestVector {
        float x;
        float y;
        float z;
    };
}

export namespace test {
    // These are causing "undefined symbol" errors in certain circumstances (see test cases in main program)
    constexpr TestVector cexpr_test_vector = TestVector{1.f,2.f,3.f};
    API TestVector dllexp_test_vector = TestVector{1.f,2.f,3.f};
    //

    API TestVector dllexp_get_test_vector() {return TestVector{1.f,2.f,3.f};}
    API void test_function(const TestVector &v)
    {
    }
}

test_program
The program links against test_library and contains several test cases which can be switched between when configuring the project with CMake (using the -DTEST_CASE=X option). Some cases trigger the linking error, others do not (see comments below for each test case).

#include <iostream>

import test_module;

// Test Case 0: Access member of constexpr vector from module test_module
// Result: Build successful
#if TEST_CASE == 0

int main() {
    std::cout<<test::cexpr_test_vector.x<<std::endl;
    return 0;
}

#endif

// Test Case 1: Call test_function from module test_module with constexpr vector
// Result: lld-link: error: undefined symbol: struct TestVector const test::cexpr_test_vector
#if TEST_CASE == 1

int main() {
    test::test_function(test::cexpr_test_vector);
    return 0;
}

#endif

// Test Case 2: Same as Test Case 1, but with locally created vector
// Result: Build successful
#if TEST_CASE == 2

int main() {
    test::test_function(TestVector {1.f, 2.f, 3.4});
    return 0;
}

#endif

// Test Case 3: Calling a locally defined function with constexpr vector as argument
// Result: Build successful
#if TEST_CASE == 3

static void print_vec(const TestVector v)
{
    std::cout<<v.x<<","<<v.y<<","<<v.z<<std::endl;
}

int main() {
    print_vec(test::cexpr_test_vector);
    return 0;
}

#endif

// Test Case 4: Same as Test Case 3, but with optimization turned off
// Result: lld-link: error: undefined symbol: struct TestVector const test::cexpr_test_vector
#if TEST_CASE == 4

#pragma clang optimize off
static void print_vec(const TestVector v)
{
    std::cout<<v.x<<","<<v.y<<","<<v.z<<std::endl;
}

int main() {
    print_vec(test::cexpr_test_vector);
    return 0;
}
#pragma clang optimize on

#endif

// Test Case 5: Same as Test Case 0, but using dllexported vector instead of constexpr
// Result: lld-link: error: undefined symbol: struct TestVector test::dllexp_test_vector
#if TEST_CASE == 5

int main() {
    std::cout<<test::dllexp_test_vector.x<<std::endl;
    return 0;
}

#endif

// Test Case 6: Same as Test Case 0, but using dllexported getter-function for vector instead of constexpr
// Result: Build successful
#if TEST_CASE == 6

int main() {
    std::cout<<test::dllexp_get_test_vector().x<<std::endl;
    return 0;
}

#endif

All test cases build and link successfully with clang on Linux.
Interestingly, when building with MSVC on Windows, Case 0 and 1 also fail with a similar error as the one clang reports:

main.cpp.obj : error LNK2019: unresolved external symbol "struct TestVector const test::cexpr_test_vector" (?cexpr_test_vector@<!-- -->test@@<!-- -->3UTestVector@@<!-- -->B::&lt;!test_module&gt;) referenced in function main

Build instructions:

Run from visual studio 2022 command prompt (replace clang executable paths CMAKE_C_COMPILER and CMAKE_CXX_COMPILER):

git clone --single-branch --branch "undefined-symbol" https://github.com/Silverlan/tmp_clang_errors.git
cd tmp_clang_errors
set root=%cd%
cd test_program
mkdir build
cd build
cmake -S .. -B . -G "Ninja" -DCMAKE_C_COMPILER="%root%/clang/bin/clang.exe"  -DCMAKE_CXX_COMPILER="%root%/clang/bin/clang++.exe" -DCMAKE_INSTALL_PREFIX:PATH="%root%/install" -DCMAKE_CXX_SCAN_FOR_MODULES=1 -DCMAKE_EXPORT_COMPILE_COMMANDS=1 -DCMAKE_BUILD_TYPE=Release -DCMAKE_POLICY_VERSION_MINIMUM=3.5 -DTEST_CASE=0
cmake --build .
cmake --install .
%root%/install/test_program.exe

(Change -DTEST_CASE=0 for each test case.)
(Alternatively you can fork the test project repository and run the GitHub workflow.)

[ChuanqiXu9]

Although I lack a windows machine, the issue itself is really nicely reduced.

Would you like to use -### to see the actual compiler drivers for the main file?

e.g., for

import test_module;
int main() {
    test::test_function(test::cexpr_test_vector);
    return 0;
}

would you like to use command line (instead of cmake) with -### to get the actual compiler flags? Similarly you can do this in linux too. And then you can share the actual compiler flags here.

Also you can use -S -emit-llvm for test_module .cppm and the main file and share the result. It is slightly shocked to me that dllexp_test_vector may be undefined too.

And also I noticed:

#ifdef _WIN32
#define API __declspec(dllexport)
#else
#define API __attribute__((visibility("default")))
#endif

And you said you've reduced as much as you can. I mean, what if you remove this part?

[Silverlan]

Although I lack a windows machine, the issue itself is really nicely reduced.

Would you like to use -### to see the actual compiler drivers for the main file?

e.g., for

import test_module;
int main() {
test::test_function(test::cexpr_test_vector);
return 0;
}

would you like to use command line (instead of cmake) with -### to get the actual compiler flags? Similarly you can do this in linux too. And then you can share the actual compiler flags here.

Also you can use -S -emit-llvm for test_module .cppm and the main file and share the result. It is slightly shocked to me that dllexp_test_vector may be undefined too.

And also I noticed:

#ifdef _WIN32
#define API __declspec(dllexport)
#else
#define API attribute((visibility("default")))
#endif

And you said you've reduced as much as you can. I mean, what if you remove this part?

Thank you for the quick response!
I would gladly provide an even further reduced example without cmake, but I really have no idea how to build a c++ project with modules without cmake.
For now I've extracted the individual build commands using the --verbose flag for cmake for test case 5 (the dllexport one).

Windows +Clang:

Run Build Command(s): C:/ProgramData/chocolatey/bin/ninja.exe -v
[1/8] C:\Windows\system32\cmd.exe /C ""D:/a/tmp_clang_errors/tmp_clang_errors/llvm-dist/bin/clang-scan-deps.exe" -format=p1689 -- D:\a\tmp_clang_errors\tmp_clang_errors\llvm-dist\bin\clang++.exe -Dtest_library_EXPORTS  -O3 -DNDEBUG -D_DLL -D_MT -Xclang --dependent-lib=msvcrt -std=gnu++23 -x c++ D:/a/tmp_clang_errors/tmp_clang_errors/test_program/test_library/test_module.cppm -c -o test_library\CMakeFiles\test_library.dir\test_module.cppm.obj -resource-dir "D:/a/tmp_clang_errors/tmp_clang_errors/llvm-dist/lib/clang/21" -MT test_library\CMakeFiles\test_library.dir\test_module.cppm.obj.ddi -MD -MF test_library\CMakeFiles\test_library.dir\test_module.cppm.obj.ddi.d > test_library\CMakeFiles\test_library.dir\test_module.cppm.obj.ddi.tmp && "C:/Program Files/CMake/bin/cmake.exe" -E rename test_library\CMakeFiles\test_library.dir\test_module.cppm.obj.ddi.tmp test_library\CMakeFiles\test_library.dir\test_module.cppm.obj.ddi"
[2/8] "C:\Program Files\CMake\bin\cmake.exe" -E cmake_ninja_dyndep --tdi=test_library\CMakeFiles\test_library.dir\CXXDependInfo.json --lang=CXX --modmapfmt=clang --dd=test_library/CMakeFiles/test_library.dir/CXX.dd @test_library/CMakeFiles/test_library.dir/CXX.dd.rsp
[3/8] D:\a\tmp_clang_errors\tmp_clang_errors\llvm-dist\bin\clang++.exe -Dtest_library_EXPORTS  -O3 -DNDEBUG -D_DLL -D_MT -Xclang --dependent-lib=msvcrt -std=gnu++23 -MD -MT test_library/CMakeFiles/test_library.dir/test_module.cppm.obj -MF test_library\CMakeFiles\test_library.dir\test_module.cppm.obj.d @test_library\CMakeFiles\test_library.dir\test_module.cppm.obj.modmap -o test_library/CMakeFiles/test_library.dir/test_module.cppm.obj -c D:/a/tmp_clang_errors/tmp_clang_errors/test_program/test_library/test_module.cppm
[4/8] C:\Windows\system32\cmd.exe /C "cd . && D:\a\tmp_clang_errors\tmp_clang_errors\llvm-dist\bin\clang++.exe -nostartfiles -nostdlib -O3 -DNDEBUG -D_DLL -D_MT -Xclang --dependent-lib=msvcrt  -fuse-ld=lld-link -shared -o test_library\test_library.dll  -Xlinker /MANIFEST:EMBED -Xlinker /implib:test_library\test_library.lib -Xlinker /pdb:test_library\test_library.pdb -Xlinker /version:0.0 test_library/CMakeFiles/test_library.dir/test_module.cppm.obj  -lkernel32 -luser32 -lgdi32 -lwinspool -lshell32 -lole32 -loleaut32 -luuid -lcomdlg32 -ladvapi32 -loldnames  && cd ."
[5/8] C:\Windows\system32\cmd.exe /C ""D:/a/tmp_clang_errors/tmp_clang_errors/llvm-dist/bin/clang-scan-deps.exe" -format=p1689 -- D:\a\tmp_clang_errors\tmp_clang_errors\llvm-dist\bin\clang++.exe -DTEST_CASE=5  -O3 -DNDEBUG -D_DLL -D_MT -Xclang --dependent-lib=msvcrt -std=gnu++23 -x c++ D:/a/tmp_clang_errors/tmp_clang_errors/test_program/main.cpp -c -o CMakeFiles\test_program.dir\main.cpp.obj -resource-dir "D:/a/tmp_clang_errors/tmp_clang_errors/llvm-dist/lib/clang/21" -MT CMakeFiles\test_program.dir\main.cpp.obj.ddi -MD -MF CMakeFiles\test_program.dir\main.cpp.obj.ddi.d > CMakeFiles\test_program.dir\main.cpp.obj.ddi.tmp && "C:/Program Files/CMake/bin/cmake.exe" -E rename CMakeFiles\test_program.dir\main.cpp.obj.ddi.tmp CMakeFiles\test_program.dir\main.cpp.obj.ddi"
[6/8] "C:\Program Files\CMake\bin\cmake.exe" -E cmake_ninja_dyndep --tdi=CMakeFiles\test_program.dir\CXXDependInfo.json --lang=CXX --modmapfmt=clang --dd=CMakeFiles/test_program.dir/CXX.dd @CMakeFiles/test_program.dir/CXX.dd.rsp
[7/8] D:\a\tmp_clang_errors\tmp_clang_errors\llvm-dist\bin\clang++.exe -DTEST_CASE=5  -O3 -DNDEBUG -D_DLL -D_MT -Xclang --dependent-lib=msvcrt -std=gnu++23 -MD -MT CMakeFiles/test_program.dir/main.cpp.obj -MF CMakeFiles\test_program.dir\main.cpp.obj.d @CMakeFiles\test_program.dir\main.cpp.obj.modmap -o CMakeFiles/test_program.dir/main.cpp.obj -c D:/a/tmp_clang_errors/tmp_clang_errors/test_program/main.cpp
[8/8] C:\Windows\system32\cmd.exe /C "cd . && D:\a\tmp_clang_errors\tmp_clang_errors\llvm-dist\bin\clang++.exe -nostartfiles -nostdlib -O3 -DNDEBUG -D_DLL -D_MT -Xclang --dependent-lib=msvcrt -Xlinker /subsystem:console  -fuse-ld=lld-link CMakeFiles/test_program.dir/main.cpp.obj -o test_program.exe -Xlinker /MANIFEST:EMBED -Xlinker /implib:test_program.lib -Xlinker /pdb:test_program.pdb -Xlinker /version:0.0   test_library/test_library.lib  -lkernel32 -luser32 -lgdi32 -lwinspool -lshell32 -lole32 -loleaut32 -luuid -lcomdlg32 -ladvapi32 -loldnames  && cd ."
FAILED: [code=1] test_program.exe 
C:\Windows\system32\cmd.exe /C "cd . && D:\a\tmp_clang_errors\tmp_clang_errors\llvm-dist\bin\clang++.exe -nostartfiles -nostdlib -O3 -DNDEBUG -D_DLL -D_MT -Xclang --dependent-lib=msvcrt -Xlinker /subsystem:console  -fuse-ld=lld-link CMakeFiles/test_program.dir/main.cpp.obj -o test_program.exe -Xlinker /MANIFEST:EMBED -Xlinker /implib:test_program.lib -Xlinker /pdb:test_program.pdb -Xlinker /version:0.0   test_library/test_library.lib  -lkernel32 -luser32 -lgdi32 -lwinspool -lshell32 -lole32 -loleaut32 -luuid -lcomdlg32 -ladvapi32 -loldnames  && cd ."
lld-link: error: undefined symbol: struct TestVector test::dllexp_test_vector
>>> referenced by CMakeFiles/test_program.dir/main.cpp.obj:(main)
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
ninja: build stopped: subcommand failed.

Linux +Clang (Successful build):

Run Build Command(s): /usr/local/bin/ninja -v
[1/8] "/home/runner/work/tmp_clang_errors/tmp_clang_errors/llvm-dist/bin/clang-scan-deps" -format=p1689 -- /home/runner/work/tmp_clang_errors/tmp_clang_errors/llvm-dist/bin/clang++ -Dtest_library_EXPORTS  -O3 -DNDEBUG -std=gnu++23 -fPIC -x c++ /home/runner/work/tmp_clang_errors/tmp_clang_errors/test_program/test_library/test_module.cppm -c -o test_library/CMakeFiles/test_library.dir/test_module.cppm.o -resource-dir "/home/runner/work/tmp_clang_errors/tmp_clang_errors/llvm-dist/lib/clang/21" -MT test_library/CMakeFiles/test_library.dir/test_module.cppm.o.ddi -MD -MF test_library/CMakeFiles/test_library.dir/test_module.cppm.o.ddi.d > test_library/CMakeFiles/test_library.dir/test_module.cppm.o.ddi.tmp && mv test_library/CMakeFiles/test_library.dir/test_module.cppm.o.ddi.tmp test_library/CMakeFiles/test_library.dir/test_module.cppm.o.ddi
[2/8] /usr/local/bin/cmake -E cmake_ninja_dyndep --tdi=test_library/CMakeFiles/test_library.dir/CXXDependInfo.json --lang=CXX --modmapfmt=clang --dd=test_library/CMakeFiles/test_library.dir/CXX.dd @test_library/CMakeFiles/test_library.dir/CXX.dd.rsp
[3/8] /home/runner/work/tmp_clang_errors/tmp_clang_errors/llvm-dist/bin/clang++ -Dtest_library_EXPORTS  -O3 -DNDEBUG -std=gnu++23 -fPIC -MD -MT test_library/CMakeFiles/test_library.dir/test_module.cppm.o -MF test_library/CMakeFiles/test_library.dir/test_module.cppm.o.d @test_library/CMakeFiles/test_library.dir/test_module.cppm.o.modmap -o test_library/CMakeFiles/test_library.dir/test_module.cppm.o -c /home/runner/work/tmp_clang_errors/tmp_clang_errors/test_program/test_library/test_module.cppm
[4/8] : && /home/runner/work/tmp_clang_errors/tmp_clang_errors/llvm-dist/bin/clang++ -fPIC -O3 -DNDEBUG  -Xlinker --dependency-file=test_library/CMakeFiles/test_library.dir/link.d -shared -Wl,-soname,libtest_library.so -o test_library/libtest_library.so test_library/CMakeFiles/test_library.dir/test_module.cppm.o   && :
[5/8] "/home/runner/work/tmp_clang_errors/tmp_clang_errors/llvm-dist/bin/clang-scan-deps" -format=p1689 -- /home/runner/work/tmp_clang_errors/tmp_clang_errors/llvm-dist/bin/clang++ -DTEST_CASE=5  -O3 -DNDEBUG -std=gnu++23 -x c++ /home/runner/work/tmp_clang_errors/tmp_clang_errors/test_program/main.cpp -c -o CMakeFiles/test_program.dir/main.cpp.o -resource-dir "/home/runner/work/tmp_clang_errors/tmp_clang_errors/llvm-dist/lib/clang/21" -MT CMakeFiles/test_program.dir/main.cpp.o.ddi -MD -MF CMakeFiles/test_program.dir/main.cpp.o.ddi.d > CMakeFiles/test_program.dir/main.cpp.o.ddi.tmp && mv CMakeFiles/test_program.dir/main.cpp.o.ddi.tmp CMakeFiles/test_program.dir/main.cpp.o.ddi
[6/8] /usr/local/bin/cmake -E cmake_ninja_dyndep --tdi=CMakeFiles/test_program.dir/CXXDependInfo.json --lang=CXX --modmapfmt=clang --dd=CMakeFiles/test_program.dir/CXX.dd @CMakeFiles/test_program.dir/CXX.dd.rsp
[7/8] /home/runner/work/tmp_clang_errors/tmp_clang_errors/llvm-dist/bin/clang++ -DTEST_CASE=5  -O3 -DNDEBUG -std=gnu++23 -MD -MT CMakeFiles/test_program.dir/main.cpp.o -MF CMakeFiles/test_program.dir/main.cpp.o.d @CMakeFiles/test_program.dir/main.cpp.o.modmap -o CMakeFiles/test_program.dir/main.cpp.o -c /home/runner/work/tmp_clang_errors/tmp_clang_errors/test_program/main.cpp
[8/8] : && /home/runner/work/tmp_clang_errors/tmp_clang_errors/llvm-dist/bin/clang++ -O3 -DNDEBUG -rdynamic  -Xlinker --dependency-file=CMakeFiles/test_program.dir/link.d CMakeFiles/test_program.dir/main.cpp.o -o test_program  -Wl,-rpath,/home/runner/work/tmp_clang_errors/tmp_clang_errors/test_program/build/test_library:  test_library/libtest_library.so && :

Unfortunately just running these commands manually does not seem to work for reproduction. I can try to create a cmake-less version and I'll report back if I can reproduce it without cmake.

Also you can use -S -emit-llvm for test_module .cppm and the main file and share the result. It is slightly shocked to me that dllexp_test_vector may be undefined too.

I hope I did it correctly, this is the command I ran:

"E:\projects\tmp_clang_errors\clang\bin\clang++.exe" ^
  -Dtest_library_EXPORTS -O3 -DNDEBUG -D_DLL -D_MT ^
  -Xclang --dependent-lib=msvcrt ^
  -std=gnu++23 -x c++ ^
  -S -emit-llvm ^
  E:/projects/tmp_clang_errors/test_program/test_library/test_module.cppm ^
  -o test_library\CMakeFiles\test_library.dir\test_module.cppm.ll ^
  -resource-dir "E:/projects/pragma_clang/deps/staging/clang/lib/clang/21"

And here are the contents of test_module.cppm.ll:

; ModuleID = 'E:/projects/tmp_clang_errors/test_program/test_library/test_module.cppm'
source_filename = "E:/projects/tmp_clang_errors/test_program/test_library/test_module.cppm"
target datalayout = "e-m:w-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
target triple = "x86_64-pc-windows-msvc19.44.35219"

%struct.TestVector = type { float, float, float }

$"??4TestVector@@QEAAAEAU0@AEBU0@@Z" = comdat any

$"??4TestVector@@QEAAAEAU0@$$QEAU0@@Z" = comdat any

@"?cexpr_test_vector@test@@3UTestVector@@B" = dso_local local_unnamed_addr constant %struct.TestVector { float 1.000000e+00, float 2.000000e+00, float 3.000000e+00 }, align 4
@"?dllexp_test_vector@test@@3UTestVector@@A" = dso_local dllexport local_unnamed_addr global %struct.TestVector { float 1.000000e+00, float 2.000000e+00, float 3.000000e+00 }, align 4

; Function Attrs: inlinehint mustprogress nounwind uwtable
define weak_odr dso_local dllexport noundef nonnull align 4 dereferenceable(12) ptr @"??4TestVector@@QEAAAEAU0@AEBU0@@Z"(ptr noundef nonnull align 4 dereferenceable(12) %0, ptr noundef nonnull align 4 dereferenceable(12) %1) local_unnamed_addr #0 comdat align 2 {
  tail call void @llvm.memcpy.p0.p0.i64(ptr noundef nonnull align 4 dereferenceable(12) %0, ptr noundef nonnull align 4 dereferenceable(12) %1, i64 12, i1 false)
  ret ptr %0
}

; Function Attrs: mustprogress nocallback nofree nounwind willreturn memory(argmem: readwrite)
declare void @llvm.memcpy.p0.p0.i64(ptr noalias writeonly captures(none), ptr noalias readonly captures(none), i64, i1 immarg) #1

; Function Attrs: inlinehint mustprogress nounwind uwtable
define weak_odr dso_local dllexport noundef nonnull align 4 dereferenceable(12) ptr @"??4TestVector@@QEAAAEAU0@$$QEAU0@@Z"(ptr noundef nonnull align 4 dereferenceable(12) %0, ptr noundef nonnull align 4 dereferenceable(12) %1) local_unnamed_addr #0 comdat align 2 {
  tail call void @llvm.memcpy.p0.p0.i64(ptr noundef nonnull align 4 dereferenceable(12) %0, ptr noundef nonnull align 4 dereferenceable(12) %1, i64 12, i1 false)
  ret ptr %0
}

; Function Attrs: mustprogress nofree norecurse nosync nounwind willreturn memory(argmem: write) uwtable
define dso_local dllexport void @"?dllexp_get_test_vector@test@@YA?AUTestVector@@XZ"(ptr dead_on_unwind noalias writable writeonly sret(%struct.TestVector) align 4 captures(none) initializes((0, 12)) %0) local_unnamed_addr #2 {
  store <2 x float> <float 1.000000e+00, float 2.000000e+00>, ptr %0, align 4
  %2 = getelementptr inbounds nuw i8, ptr %0, i64 8
  store float 3.000000e+00, ptr %2, align 4
  ret void
}

; Function Attrs: mustprogress nofree norecurse nosync nounwind willreturn memory(none) uwtable
define dso_local dllexport void @"?test_function@test@@YAXAEBUTestVector@@@Z"(ptr noundef nonnull readnone align 4 captures(none) dereferenceable(12) %0) local_unnamed_addr #3 {
  ret void
}

attributes #0 = { inlinehint mustprogress nounwind uwtable "min-legal-vector-width"="0" "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cmov,+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }
attributes #1 = { mustprogress nocallback nofree nounwind willreturn memory(argmem: readwrite) }
attributes #2 = { mustprogress nofree norecurse nosync nounwind willreturn memory(argmem: write) uwtable "min-legal-vector-width"="0" "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cmov,+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }
attributes #3 = { mustprogress nofree norecurse nosync nounwind willreturn memory(none) uwtable "min-legal-vector-width"="0" "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cmov,+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }

!llvm.dbg.cu = !{!0}
!llvm.linker.options = !{!2}
!llvm.module.flags = !{!3, !4, !5, !6, !7}
!llvm.ident = !{!8}

!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, producer: "clang version 21.1.8", isOptimized: true, runtimeVersion: 0, emissionKind: NoDebug, splitDebugInlining: false, nameTableKind: None)
!1 = !DIFile(filename: "E:/projects/tmp_clang_errors/test_program/test_library\\test_module.cppm", directory: "E:\\projects\\tmp_clang_errors\\test_program\\build")
!2 = !{!"/DEFAULTLIB:msvcrt.lib"}
!3 = !{i32 2, !"Debug Info Version", i32 3}
!4 = !{i32 1, !"wchar_size", i32 2}
!5 = !{i32 8, !"PIC Level", i32 2}
!6 = !{i32 7, !"uwtable", i32 2}
!7 = !{i32 1, !"MaxTLSAlign", i32 65536}
!8 = !{!"clang version 21.1.8"}

If I try to do the same for main.cpp, I get:

E:/projects/tmp_clang_errors/test_program/main.cpp:3:8: fatal error: module 'test_module' not found
    3 | import test_module;
      | ~~~~~~~^~~~~~~~~~~
1 error generated.

I'll play around with it some more and see if I can get the output.

And you said you've reduced as much as you can. I mean, what if you remove this part?

You mean without the use of a macro?
I have tried this:

export module test_module;

export {
    struct __declspec(dllexport) TestVector {
        float x;
        float y;
        float z;
    };
}

export namespace test {
    constexpr TestVector cexpr_test_vector = TestVector{1.f,2.f,3.f};
    __declspec(dllexport) TestVector dllexp_test_vector = TestVector{1.f,2.f,3.f};
    __declspec(dllexport) TestVector dllexp_get_test_vector() {return TestVector{1.f,2.f,3.f};}

    __declspec(dllexport) void test_function(const TestVector &v)
    {
    }
}

but the result is the same:
lld-link: error: undefined symbol: struct TestVector test::dllexp_test_vector

Removing __declspec(dllexport) altogether also produces the same result for test case 5 (which makes sense of course):

export module test_module;

export {
    struct  TestVector {
        float x;
        float y;
        float z;
    };
}

export namespace test {
    constexpr TestVector cexpr_test_vector = TestVector{1.f,2.f,3.f};
     TestVector dllexp_test_vector = TestVector{1.f,2.f,3.f};
     TestVector dllexp_get_test_vector() {return TestVector{1.f,2.f,3.f};}

     void test_function(const TestVector &v)
    {
    }
}

// Edit:
Just to make sure, I've also tested it on the latest trunk version of clang (at commit faf140a) and the error is still there.

[Silverlan]

@ChuanqiXu9 I have added a branch without cmake to the test project repository:
https://github.com/Silverlan/tmp_clang_errors/tree/undefined-symbol-no-cmake

Build commands (windows):

cd test_program
clang++.exe -std=c++20 test_module/test_module.cppm --precompile -o test_module/test_module.pcm
clang++.exe -std=c++20 test_module/test_module.pcm -c -o test_module/test_module.o
clang++.exe -fuse-ld=lld -shared -o test_module/test_module.dll test_module/test_module.o -Wl,/IMPLIB:test_module/test_module.lib
clang++.exe -std=c++20 main.cpp -fprebuilt-module-path=test_module -c -o main.o -DTEST_CASE=%1
clang++.exe -fuse-ld=lld main.o test_module/test_module.lib -o app.exe

Build commands (linux):

cd test_program
clang++ -std=c++20 test_module/test_module.cppm --precompile -o test_module/test_module.pcm
clang++ -std=c++20 test_module/test_module.pcm -c -o test_module/test_module.o
clang++ -shared -o test_module/libtest_module.so test_module/test_module.o
clang++ -std=c++20 main.cpp -fprebuilt-module-path=test_module -c -o main.o -DTEST_CASE=0
clang++ main.o test_module/libtest_module.so -o app

(Adjust -DTEST_CASE for the respective test case.)
(There is also a convenience build.bat for windows and build.sh for linux in the "test_program" directory. You can pass the test case id as argument.)

I've updated the workflows for this branch accordingly, here are the logs:
Windows with clang: https://github.com/Silverlan/tmp_clang_errors/actions/runs/20605776429/job/59181152772
Linux with clang: https://github.com/Silverlan/tmp_clang_errors/actions/runs/20605776428/job/59181152736

The end result for each test is the same as with the cmake version, except for test case 3, which now also fails (this one was successful in the came version).

[ChuanqiXu9]

but I really have no idea how to build a c++ project with modules without cmake.

if you want, you can look at https://clang.llvm.org/docs/StandardCPlusPlusModules.html

[ChuanqiXu9]

@ChuanqiXu9 I have added a branch without cmake to the test project repository: https://github.com/Silverlan/tmp_clang_errors/tree/undefined-symbol-no-cmake

Build commands (windows):

cd test_program
clang++.exe -std=c++20 test_module/test_module.cppm --precompile -o test_module/test_module.pcm
clang++.exe -std=c++20 test_module/test_module.pcm -c -o test_module/test_module.o
clang++.exe -fuse-ld=lld -shared -o test_module/test_module.dll test_module/test_module.o -Wl,/IMPLIB:test_module/test_module.lib
clang++.exe -std=c++20 main.cpp -fprebuilt-module-path=test_module -c -o main.o -DTEST_CASE=%1
clang++.exe -fuse-ld=lld main.o test_module/test_module.lib -o app.exe

Build commands (linux):

cd test_program
clang++ -std=c++20 test_module/test_module.cppm --precompile -o test_module/test_module.pcm
clang++ -std=c++20 test_module/test_module.pcm -c -o test_module/test_module.o
clang++ -shared -o test_module/libtest_module.so test_module/test_module.o
clang++ -std=c++20 main.cpp -fprebuilt-module-path=test_module -c -o main.o -DTEST_CASE=0
clang++ main.o test_module/libtest_module.so -o app

(Adjust -DTEST_CASE for the respective test case.) (There is also a convenience build.bat for windows and build.sh for linux in the "test_program" directory. You can pass the test case id as argument.)

I've updated the workflows for this branch accordingly, here are the logs: Windows with clang: https://github.com/Silverlan/tmp_clang_errors/actions/runs/20605776429/job/59181152772 Linux with clang: https://github.com/Silverlan/tmp_clang_errors/actions/runs/20605776428/job/59181152736

The end result for each test is the same as with the cmake version, except for test case 3, which now also fails (this one was successful in the came version).

Yeah, this is what expected. Then you can use -### to get the actual compile flags on windows and linux. And also -S -emit-llvm

[ChuanqiXu9]

And look at the test_module.cppm.ll you pasted, we can find:

@"?cexpr_test_vector@test@@3UTestVector@@B" = dso_local local_unnamed_addr constant %struct.TestVector { float 1.000000e+00, float 2.000000e+00, float 3.000000e+00 }, align 4
@"?dllexp_test_vector@test@@3UTestVector@@A" = dso_local dllexport local_unnamed_addr global %struct.TestVector { float 1.000000e+00, float 2.000000e+00, float 3.000000e+00 }, align 4

the @"?cexpr_test_vector@test@@3UTestVector@@B" and @"?dllexp_test_vector@test@@3UTestVector@@A" are the missing symbol. The local_unnamed_addr here seems problematic.

Then you can try to see the symbols for the .o file and the .dll file to see if these symbols are provided. I don't know the concrete command in windows. You can search it for windows. In linux, we can get them by nm.

[Silverlan]

Yeah, this is what expected. Then you can use -### to get the actual compile flags on windows and linux. And also -S -emit-llvm

Here is the -### output for the main.cpp:

clang version 21.1.8
Target: x86_64-pc-windows-msvc
Thread model: posix
InstalledDir: D:\a\tmp_clang_errors\tmp_clang_errors\llvm-dist\bin
 (in-process)
 "D:\\a\\tmp_clang_errors\\tmp_clang_errors\\llvm-dist\\bin\\clang++.exe" "-cc1" "-triple" "x86_64-pc-windows-msvc19.44.35222" "-emit-obj" "-mincremental-linker-compatible" "-disable-free" "-clear-ast-before-backend" "-disable-llvm-verifier" "-discard-value-names" "-main-file-name" "main.cpp" "-mrelocation-model" "pic" "-pic-level" "2" "-mframe-pointer=none" "-relaxed-aliasing" "-fmath-errno" "-ffp-contract=on" "-fno-rounding-math" "-mconstructor-aliases" "-fms-volatile" "-funwind-tables=2" "-target-cpu" "x86-64" "-tune-cpu" "generic" "-fdebug-compilation-dir=D:\\a\\tmp_clang_errors\\tmp_clang_errors\\test_program" "-fcoverage-compilation-dir=D:\\a\\tmp_clang_errors\\tmp_clang_errors\\test_program" "-resource-dir" "D:\\a\\tmp_clang_errors\\tmp_clang_errors\\llvm-dist\\lib\\clang\\21" "-D" "TEST_CASE=5" "-internal-isystem" "D:\\a\\tmp_clang_errors\\tmp_clang_errors\\llvm-dist\\lib\\clang\\21\\include" "-internal-isystem" "C:\\Program Files\\Microsoft Visual Studio\\2022\\Enterprise\\VC\\Tools\\MSVC\\14.44.35207\\include" "-internal-isystem" "C:\\Program Files\\Microsoft Visual Studio\\2022\\Enterprise\\VC\\Tools\\MSVC\\14.44.35207\\atlmfc\\include" "-internal-isystem" "C:\\Program Files (x86)\\Windows Kits\\10\\Include\\10.0.26100.0\\ucrt" "-internal-isystem" "C:\\Program Files (x86)\\Windows Kits\\10\\Include\\10.0.26100.0\\shared" "-internal-isystem" "C:\\Program Files (x86)\\Windows Kits\\10\\Include\\10.0.26100.0\\um" "-internal-isystem" "C:\\Program Files (x86)\\Windows Kits\\10\\Include\\10.0.26100.0\\winrt" "-internal-isystem" "C:\\Program Files (x86)\\Windows Kits\\10\\Include\\10.0.26100.0\\cppwinrt" "-std=c++20" "-fdeprecated-macro" "-ferror-limit" "19" "-fno-use-cxa-atexit" "-fms-extensions" "-fms-compatibility" "-fms-compatibility-version=19.44.35222" "-fno-implicit-modules" "-fprebuilt-module-path=test_module" "-fskip-odr-check-in-gmf" "-fcxx-exceptions" "-fexceptions" "-faddrsig" "-o" "main.o" "-x" "c++" "main.cpp"

And here is the -### output for test_module.cppm:

clang version 21.1.8
Target: x86_64-pc-windows-msvc
Thread model: posix
InstalledDir: D:\a\tmp_clang_errors\tmp_clang_errors\llvm-dist\bin
 (in-process)
 "D:\\a\\tmp_clang_errors\\tmp_clang_errors\\llvm-dist\\bin\\clang++.exe" "-cc1" "-triple" "x86_64-pc-windows-msvc19.44.35222" "-emit-module-interface" "-disable-free" "-clear-ast-before-backend" "-disable-llvm-verifier" "-discard-value-names" "-main-file-name" "test_module.cppm" "-mrelocation-model" "pic" "-pic-level" "2" "-mframe-pointer=none" "-relaxed-aliasing" "-fmath-errno" "-ffp-contract=on" "-fno-rounding-math" "-mconstructor-aliases" "-fms-volatile" "-funwind-tables=2" "-target-cpu" "x86-64" "-tune-cpu" "generic" "-fdebug-compilation-dir=D:\\a\\tmp_clang_errors\\tmp_clang_errors\\test_program" "-fcoverage-compilation-dir=D:\\a\\tmp_clang_errors\\tmp_clang_errors\\test_program" "-resource-dir" "D:\\a\\tmp_clang_errors\\tmp_clang_errors\\llvm-dist\\lib\\clang\\21" "-internal-isystem" "D:\\a\\tmp_clang_errors\\tmp_clang_errors\\llvm-dist\\lib\\clang\\21\\include" "-internal-isystem" "C:\\Program Files\\Microsoft Visual Studio\\2022\\Enterprise\\VC\\Tools\\MSVC\\14.44.35207\\include" "-internal-isystem" "C:\\Program Files\\Microsoft Visual Studio\\2022\\Enterprise\\VC\\Tools\\MSVC\\14.44.35207\\atlmfc\\include" "-internal-isystem" "C:\\Program Files (x86)\\Windows Kits\\10\\Include\\10.0.26100.0\\ucrt" "-internal-isystem" "C:\\Program Files (x86)\\Windows Kits\\10\\Include\\10.0.26100.0\\shared" "-internal-isystem" "C:\\Program Files (x86)\\Windows Kits\\10\\Include\\10.0.26100.0\\um" "-internal-isystem" "C:\\Program Files (x86)\\Windows Kits\\10\\Include\\10.0.26100.0\\winrt" "-internal-isystem" "C:\\Program Files (x86)\\Windows Kits\\10\\Include\\10.0.26100.0\\cppwinrt" "-std=c++20" "-fdeprecated-macro" "-ferror-limit" "19" "-fno-use-cxa-atexit" "-fms-extensions" "-fms-compatibility" "-fms-compatibility-version=19.44.35222" "-fno-implicit-modules" "-fskip-odr-check-in-gmf" "-fcxx-exceptions" "-fexceptions" "-faddrsig" "-o" "test_module/test_module.pcm" "-x" "c++" "test_module/test_module.cppm"

I've uploaded the files generated by -S -emit-llvm, as some of them are fairly large:
debug-files.zip
test_module.cppm -> test_module.ll
main.cpp -> main_case0.ll, etc. (one for each case)

Then you can try to see the symbols for the .o file and the .dll file to see if these symbols are provided. I don't know the concrete command in windows. You can search it for windows. In linux, we can get them by nm.

The results using dumpbin /EXPORTS test_module.dll:

Microsoft (R) COFF/PE Dumper Version 14.44.35219.0
Copyright (C) Microsoft Corporation.  All rights reserved.


Dump of file case1\test_module.dll

File Type: DLL

  Section contains the following exports for test_module.dll

    00000000 characteristics
           0 time date stamp
        0.00 version
           1 ordinal base
           5 number of functions
           5 number of names

    ordinal hint RVA      name

          1    0 00001040 ??4TestVector@@QEAAAEAU0@$$QEAU0@@Z
          2    1 00001040 ??4TestVector@@QEAAAEAU0@AEBU0@@Z
          3    2 00001000 ?dllexp_get_test_vector@test@@YA?AUTestVector@@XZ
          4    3 00019000 ?dllexp_test_vector@test@@3UTestVector@@A
          5    4 00001030 ?test_function@test@@YAXAEBUTestVector@@@Z

  Summary

        2000 .data
        1000 .fptable
        1000 .pdata
        A000 .rdata
        1000 .reloc
        E000 .text
        1000 _RDATA

?dllexp_test_vector@test@@3UTestVector@@A is defined, ?cexpr_test_vector@test@@3UTestVector@@B is not.

// Edit:
And for the .o file:

dumpbin /symbols test_module.o
Microsoft (R) COFF/PE Dumper Version 14.44.35219.0
Copyright (C) Microsoft Corporation.  All rights reserved.


Dump of file test_module.o

File Type: COFF OBJECT

COFF SYMBOL TABLE
000 00000000 SECT1  notype       Static       | .text
    Section length   37, #relocs    3, #linenums    0, checksum F16C8C06
002 00000000 SECT2  notype       Static       | .data
    Section length    C, #relocs    0, #linenums    0, checksum C9DB50DE
004 00000000 SECT3  notype       Static       | .bss
    Section length    0, #relocs    0, #linenums    0, checksum        0
006 00000000 SECT4  notype       Static       | .rdata
    Section length    4, #relocs    0, #linenums    0, checksum 86A50E95, selection    2 (pick any)
008 00000000 SECT4  notype       External     | __real@40400000
009 00000000 SECT5  notype       Static       | .rdata
    Section length    4, #relocs    0, #linenums    0, checksum 76DC4190, selection    2 (pick any)
00B 00000000 SECT5  notype       External     | __real@40000000
00C 00000000 SECT6  notype       Static       | .rdata
    Section length    4, #relocs    0, #linenums    0, checksum 8DE5B576, selection    2 (pick any)
00E 00000000 SECT6  notype       External     | __real@3f800000
00F 00000000 SECT7  notype       Static       | .xdata
    Section length   10, #relocs    0, #linenums    0, checksum 53EC3525
011 00000000 SECT8  notype       Static       | .text
    Section length   27, #relocs    0, #linenums    0, checksum  BD30D19, selection    2 (pick any)
013 00000000 SECT8  notype ()    External     | ??4TestVector@@QEAAAEAU0@AEBU0@@Z (public: struct TestVector & __cdecl TestVector::operator=(struct TestVector const &))
014 00000000 SECTF  notype       Static       | .xdata
    Section length    8, #relocs    0, #linenums    0, checksum 63791761, selection    5 (pick associative Section 0x8)
016 00000000 SECT9  notype       Static       | .text
    Section length   27, #relocs    0, #linenums    0, checksum  BD30D19, selection    2 (pick any)
018 00000000 SECT9  notype ()    External     | ??4TestVector@@QEAAAEAU0@$$QEAU0@@Z (public: struct TestVector & __cdecl TestVector::operator=(struct TestVector &&))
019 00000000 SECT10 notype       Static       | .xdata
    Section length    8, #relocs    0, #linenums    0, checksum 63791761, selection    5 (pick associative Section 0x9)
01B 00000000 SECTA  notype       Static       | .rdata
    Section length    C, #relocs    0, #linenums    0, checksum C9DB50DE
01D 00000000 SECTB  notype       Static       | .drectve
    Section length  104, #relocs    0, #linenums    0, checksum 73F0B021
01F 00000000 SECTC  notype       Static       | .debug$S
    Section length   48, #relocs    0, #linenums    0, checksum FC36E3EC
021 00000000 SECTD  notype       Static       | .pdata
    Section length   18, #relocs    6, #linenums    0, checksum A4AFBFA1
023 00000000 SECT11 notype       Static       | .pdata
    Section length    C, #relocs    3, #linenums    0, checksum F3B36B4F, selection    5 (pick associative Section 0x8)
025 00000000 SECT12 notype       Static       | .pdata
    Section length    C, #relocs    3, #linenums    0, checksum F3B36B4F, selection    5 (pick associative Section 0x9)
027 00000000 SECTE  notype       Static       | .llvm_addrsig
    tag index 00000000 size 00000000 lines 00000000 next function 0000000E
029 00000000 ABS    notype       Static       | @feat.00
02A 00000000 SECT1  notype ()    External     | ?dllexp_get_test_vector@test@@YA?AUTestVector@@XZ (struct TestVector __cdecl test::dllexp_get_test_vector(void))
02B 00000030 SECT1  notype ()    External     | ?test_function@test@@YAXAEBUTestVector@@@Z (void __cdecl test::test_function(struct TestVector const &))
02C 00000000 SECTA  notype       External     | ?cexpr_test_vector@test@@3UTestVector@@B (struct TestVector const test::cexpr_test_vector)
02D 00000000 SECT2  notype       External     | ?dllexp_test_vector@test@@3UTestVector@@A (struct TestVector test::dllexp_test_vector)
02E 00000000 UNDEF  notype       External     | _fltused
02F 00000000 DEBUG  notype       Filename     | .file
    test_module.pcm

String Table Size = 0x138 bytes

  Summary

           0 .bss
           C .data
          48 .debug$S
         104 .drectve
           0 .llvm_addrsig
          30 .pdata
          18 .rdata
          85 .text
          20 .xdata

Here both symbols are defined.