How do I use C++ modules in Clang?

Like you mentioned, clang does not yet have a C++ syntax for imports, so I doubt that #include directives are going to be literally rewritten as imports when preprocessing a file, so that may not be the best way to test if modules are working as intended.

However, if you set -fmodules-cache-path=<path> explicitly, you can observe clang populating it with precompiled module files (*.pcm) during a build - if there are any modules involved.

You'll need to use libc++ (which seems to come with a module.modulemap as of version 3.7.0) if you want to use a modules enabled standard library right now - though in my experience this isn't working entirely just yet. (Visual Studio 2015's C++ compiler is also supposed to get some form of module support with Update 1 in November)

Independently of the stdlib, you could still use modules in your own code. The clang docs contain a detailed description of the Module Map Language.

As of this commit, Clang has experimental support for the Modules TS.

Let's take the same example files (with a small change) as in the VS blog post about experimental module support.

First, define the module interface file. By default, Clang recognizes files with cppm extension (and some others) as C++ module interface files.

// file: foo.cppm
export module M;

export int f(int x)
{
    return 2 + x;
}
export double g(double y, int z)
{
    return y * z;
} 

Note that the module interface declaration needs to be export module M; and not just module M; like in the VS blog post.

Then consume the module as follows:

// file: bar.cpp
import M;

int main()
{
    f(5);
    g(0.0, 1);
    return 0;
}

Now, precompile the module foo.cppm with

clang++ -fmodules-ts --precompile foo.cppm -o M.pcm

or, if the module interface extension is other than cppm (let's say ixx, as it is with VS), you can use:

clang++ -fmodules-ts --precompile -x c++-module foo.ixx -o M.pcm

Then build the program with

clang++ -fmodules-ts -c M.pcm -o M.o
clang++ -fmodules-ts -fprebuilt-module-path=. M.o bar.cpp

or, if the pcm file name is not the same as the module name, you'd have to use:

clang++ -fmodules-ts -fmodule-file=M.pcm bar.cpp

I've tested these commands on Windows using the r303050 build (15th May 2017).

Note: When using the -fprebuilt-module-path=. option, I get a warning:

clang++.exe: warning: argument unused during compilation: '-fprebuilt-module-path=.' [-Wunused-command-line-argument]

which appears to be incorrect because without that option, the module M is not found.