[Issue 19400] New: Very brittle undefined symbol linker failure when importing modules

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19400

          Issue ID: 19400
           Summary: Very brittle undefined symbol linker failure when
                    importing modules
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: druntime
          Assignee: nobody at puremagic.com
          Reporter: n8sh.secondary at hotmail.com

Created attachment 1717
  --> https://issues.dlang.org/attachment.cgi?id=1717&action=edit
demonstrates the bug

Attached is a test demonstrating the bug. The following works: `dmd bug1.d
includes/bug2.d`. The following fails in linking: `dmd bug1.d -I=./includes`.
The error message is:

Undefined symbols for architecture x86_64:
  "__D6object__T8__equalsTxC6ObjectTxQkZQyFAxQsQeZb", referenced from:
     
__D4bug2__T5DummySQp__T6StructTvZQk3fooMFiZ9__lambda2ZQBr11__xopEqualsFKxSQCt__TQCrSQDd__TQCpTvZQCvQCmMFiZQClZQDvKxQBqZb
in b


The source code split between two files is:

--- bug1.d
module bug1;
import bug2 : Struct;

size_t foo(ref Struct!void e)
{
    return cast(size_t) &e;
}

void main()
{
}

--- includes/bug2.d

module bug2;

private struct Dummy(alias pred)
{
    bool hasAny;

    private Object[] _; // Compilation succeeds if you remove this.
}

struct Struct(Unused)
{
    bool foo(int f)
    {
        return Dummy!(a => a == f).init.hasAny;
    }

    private Object[] _; // Compilation succeeds if you remove this.
}

private struct Unused2
{
    Struct!void x;
}
---

--