Using mixin from different modules is not working

[apz28]

--- test1.d

mixin template C(bool isBase)
{
    this(int x)
    {
        static if (!isBase)
        {
            super(x);
        }
    }
}

class B
{
public:
    mixin C!true;
}

--- test2.d

import test1;

mixin template C2(bool isBase)
{
    this(int x, string y)
    {
        static if (isBase)
        {
            super(x);
        }
        else
        {
            super(x, y);
        }
    }
}

class E : B
{
public:
    mixin C!false;
    mixin C2!true;
}

--- test3.d

import test1;
import test2;

void main()
{
    auto b = new B(0);
    auto e = new E(0, "");
}

Error as below from online compiler - https://run.dlang.io/?compiler=dmd
If merge test2 to test1, no error taken place

/usr/lib/gcc/x86_64-linux-gnu/13/../../../x86_64-linux-gnu/Scrt1.o:function _start:(.text+0x1b): error: undefined reference to 'main'
test2.o:/sandbox/test2.d:_D5test21E7__ClassZ:(.data._D5test21E7__ClassZ+0x50): error: undefined reference to '_D5test11B7__ClassZ'
./test1.d:7: error: undefined reference to '_D5test11B8__mixin16__ctorMFiZCQBdQBa'
/sandbox/test2.d:9: error: undefined reference to '_D5test11B8__mixin16__ctorMFiZCQBdQBa'
collect2: error: ld returned 1 exit status
Error: undefined reference to `main`
       referenced from `_start`
Error: undefined reference to `test1.B.__Class`
Error: undefined reference to `test1.B test1.B.__mixin1.__ctor(int)`
Error: undefined reference to `test1.B test1.B.__mixin1.__ctor(int)`
       perhaps define a `void main() {}` function or use the `-main` switch
Error: linker exited with status 1
       cc test2.o -o test2 -g -m64 -Xlinker --export-dynamic -L/dlang/dmd/linux/bin64/../lib64 -Xlinker -Bstatic -lphobos2 -Xlinker -Bdynamic -lpthread -lm -lrt -ldl 

[Zish19]

@apz28
After investigating this locally, I wasn't able to reproduce a compiler bug.

Compiling the reproducer with DMD using:

dmd test1.d test2.d test3.d

works correctly. The mixin-generated constructor is emitted in the object file for test1.d, referenced from test2.d, and linked successfully.

I then investigated the run.dlang.io environment referenced in the report. From the backend invocation and the linker output shown in the issue:

cc test2.o -o test2 ...

it appears that only test2.o reaches the final link step. As a result, symbols defined in test1.o (including test1.B.__Class and the mixin-generated constructor) are unavailable, producing the reported undefined references.

Based on this investigation, the compiler itself appears to behave correctly when all modules are compiled and linked together. The observed failure seems to be related to how the online environment invokes DMD for multi-file snippets rather than to mixin instantiation or cross-module code generation.

If I've understood the execution environment correctly, this may be more appropriate as an issue against the run.dlang.io infrastructure than against DMD itself. I'd be interested to hear whether the maintainers agree with that assessment.