adaptor-mod

This test C++ "solution" for Visual Studio 2026 (open the file adaptor-mod.slnx) explores what happens, if two libraries have a module with the same name (X).

The main project builds a Windows exe, which returns a value (file main/main.cpp):

import XA;
import XB;

int main() {
    auto z1 = XA::Z{};
    auto z2 = XB::Z{};
    z1.add();
    z2.add();
    auto sum = z1.i + z2.i;
    return sum;
}

It imports two modules: XA and XB.

Module XA is defined in library XA (file XA/XA.ixx):

export module XA;

import X;  // from library A

export namespace XA
{

using Z = X::Z;

}

XA imports module X, which is defined in library A (file A/X.ixx):

export module X;  // library A

namespace X
{

export struct Z
{
    int i = 42;
    void add()
    {
        i = i + 7;
    }
};

}

Module XB is defined in library XB (file XB/XB.ixx):

export module XB;

import X;  // from library B

export namespace XB
{

using Z = X::Z;

}

XB imports module X, which is defined in library B (file B/X.ixx):

export module X;  // library B

namespace X
{

export struct Z
{
    int i = 3;
    void add()
    {
        i = i + 17;
    }
};

}

So, both libraries A and B have a module with the same name X, but the contents are not the same. They both have a struct Z, but the definitions differ.

Note that MSVC finds modules by using references to "projects". Project XB (which builds the library XB) references "project" B. So when building XB, the compiler searches for module X in project B. When building XA, the compiler searches for module X in project A.