| Issue |
174858
|
| Summary |
[C++ Modules] "import std" conflict with module that includes STL headers: std::println compilation failure
|
| Labels |
libc++,
clang:modules
|
| Assignees |
|
| Reporter |
kaladron
|
I am encountering a compilation error on x86_64 Clang 21 and Clang Trunk when using import std; alongside a user-defined named module. The user-defined module includes 3 STL headers, and this causes a resolution failure in the main test file.
Github won't let me attach .cc or .cppm files, so I'm including them inline.
[CMakeLists.txt](https://github.com/user-attachments/files/24481535/CMakeLists.txt)
test.cc:
```
import std;
import header_wrapper;
int main() {
// This should work but causes compilation errors with LLVM
std::println("Testing std::println after importing header_wrapper");
return 0;
}
```
header_wrapper.cppm:
```
// Module that wraps the header library
module;
#include <chrono>
#include <cstddef>
#include <ostream>
export module header_wrapper;
export void f1(std::size_t);
```
These seem to be the relevant errors during compilation:
```
In module 'std' imported from /app/test.cc:1:
.../format_functions.h:99:30: error: call to implicitly-deleted default constructor of 'formatter<std::__1::basic_format_string<char>, char>'
99 | formatter<_Tp, _CharT> __f;
...
test.cc:6:7: note: in instantiation of function template specialization 'std::__1::println<>' requested here
6 | std::println("Testing std::println after importing header_wrapper");
```
But I've also attached the full build error:
[buildlog.txt](https://github.com/user-attachments/files/24482026/buildlog.txt)
h/t to @dwblaikie who helped me further reduce my test case and helped me with godbolt: https://godbolt.org/z/GMaaqvPrK where we tested this with Clang 21 and Clang trunk.
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