Modules Header Unit Multiple Definition

[CubedProgrammer]

This following code.

import <sstream>;
import <string>;
std::string s;

Compiled with

clang++ -std=c++20 -stdlib=libc++ -fmodule-file=sstream.pcm -fmodule-file=string.pcm -c PROGRAM.cc

Will produce a multiple definition error, saying that string is defined twice.

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: CubedProgrammer (CubedProgrammer)

This following code. ```cpp import <sstream>; import <string>; std::string s; ``` Compiled with ``` clang++ -std=c++20 -stdlib=libc++ -fmodule-file=sstream.pcm -fmodule-file=string.pcm -c PROGRAM.cc ``` Will produce a multiple definition error, saying that string is defined twice.

[CubedProgrammer]

Update to the bug:

I was unable to replicate this bug using my own header files. However, I have found a way to make the error go away.

Compile the file __fwd/string.h into a pcm file.

Compile the file string into a pcm file, with the pcm file of __fwd/string.h specified on the command line.

Compile the file sstream into a pcm file, with the pcm file of __fwd/string.h specified on the command line.

Compile the sample code shown in the original post with all three pcm files.

clang++ -std=c++20 -stdlib=libc++ --precompile -x c++-system-header __fwd/string.h -o __fwd-string.pcm
clang++ -std=c++20 -stdlib=libc++ --precompile -fmodule-file=__fwd-string.pcm -x c++-system-header string -o string.pcm
clang++ -std=c++20 -stdlib=libc++ --precompile -fmodule-file=__fwd-string.pcm -x c++-system-header sstream -o sstream.pcm
clang++ -std=c++20 -stdlib=libc++ -fmodule-file=__fwd-string.pcm -fmodule-file=string.pcm -fmodule-file=sstream.pcm -c import_multiple_definition.cc

Note that the sstream file doesn't actually need the fwd string specified on the command line, my theory is that every single header that includes __fwd/string.h needs it specified on the command line, except for one. I may test that theory soon, since I do need it for a real project.

[CubedProgrammer]

Update on the last statement of the previous comment. Confirmed for string_view, there are some other types that do this in the standard library, not sure why those specifically.

For every header the base header's pcm is specified on the command line during its compilation, it will remove one duplicate definition.

[CubedProgrammer]

Update again: While attempting to build my project, the error occurred with nlohmann/json, I dug a little deeper and I believe the error is caused by declaring a class template and making an alias for a specialization of it. However, I was still unable to replicate it using my own header files.