import std.stdio;

struct S
{
    int i;
    void bar()
    {
        ++i;
    }
    void foo() immutable
    {
        void delegate() dg1 = &bar;
        dg1(); // comment out to fix
        
        void delegate() dg2;
        dg2.ptr = cast(void*)&this;
        dg2.funcptr = cast(void function())&S.bar;
        dg2();
    }
}

void main()
{
    immutable S s;
    writeln(s.i);
    s.foo();
    writeln(s.i);
}

Without dg1() call, dmd arranges registers's content correctly. However in presence of dg1 call it incorrectly rearranges them so that [rex.W callq] calls this pointer, not function pointer.