C++-module import chain results in strange compiler errors

Question

When compiling c++-20 modules with clang. I get strange compilation errors when importing a file that imports another file that imports a third file like so:

// a.cppm
#include <string>

export module a;

export std::string getStuffA() {
  return "a";
}

// b.cppm
#include <string>

import a;

export module b;

export std::string getStuffB() {
  return "b" + getStuffA();
}

// c.cpp
#include <string>

import b;

int main() {
  std::cout << getStuffB() << std::endl;
  return 0;
}

The compiler outputs:

In file included from ./src/c.cpp:1:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/iostream:39:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/ostream:38:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/ios:44:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/basic_ios.h:37:
/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/locale_facets.h:2569:5: error: inline declaration of 'isspace' follows non-inline definition

And it lists some more functions that seems to have both inline and non-inline definiton, like isprint, iscntrl, isupper.

Is there any way to get around these errors.

Note: Removing std::string and #include <string> removes the compilation errors, so I think that the errors comes from <string>

Module support looks like it needs gcc 11 see https://gcc.gnu.org/projects/cxx-status.html — Richard Critten, Feb 28 '21 at 17:34
Am not sure; compiler support (just language support) is not the same as Standard Library support - you are going to need to check both. — Richard Critten, Feb 28 '21 at 17:38
See also https://stackoverflow.com/questions/66411157/import-std-lib-as-modules-with-clang _"...The C++20 standard does not include module definitions for the C++ standard library..."_ — Richard Critten, Feb 28 '21 at 18:05
@RichardCritten No I know, but you must be able to use the standard library somhow. — Lasersköld, Feb 28 '21 at 19:50
@S.M. Yes no prop found it with some googling. That solves the problem. Thanks! I guess that if you write it as a regular answer i would accept that :) — Lasersköld, Feb 28 '21 at 20:28
@S.M. Fixed. Tried to throw together a minimal viable example in the editor... — Lasersköld, Feb 28 '21 at 20:47

score 0 · Accepted Answer · answered Feb 28 '21 at 20:41

As already said, modules are supported in gcc 11, and you are trying to use clang with gcc 10 standard C++ library.

With using the proper C++ standard library, preprocessor include directives such as #include <string> will be automatically mapped to import <string>;. With using the standard C++ library for gcc 10, that does not support modules, you will get errors such as you have got.

If you use clang, that supports modules, and it seems you are using such modern clang, otherwise you would get another errors on the import directive, you should use the clang C++ standard library: -stdlib=libc++.

C++-module import chain results in strange compiler errors

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