reduced case.  Requires -m64 and -O.  Without -O it passes:

module bug;

extern(C) int printf(const char*, ...);

wstring toImpl(int value)
{
    wchar[13] buffer;

    auto u = -cast(int) value;
    uint ndigits = 1;
    while (u)
    {  
        immutable c = cast(char)((u % 10) + '0');
        u /= 10;
        buffer[$ - ndigits] = c;
        ++ndigits;
    }
    assert(ndigits <= buffer.length);
    buffer[$ - ndigits] = '-';
    //printf("ndigits = %d, buffer.length = %zd, diff = %d\n", ndigits, buffer.length, buffer.length - ndigits);
    return cast(wstring) buffer[buffer.length - ndigits .. buffer.length].dup;
}

int main()
{
    wstring s = toImpl(int.min);
    return 0;
}

Uncommenting the printf results in:
    Illegal instruction

from objdump --disassemble:
  fe:   66 48 c7 06 2d 00 48    data32 movq $0xffffffff8948002d,(%rsi)
 105:   89 
 106:   d9 48 ba                (bad)  -0x46(%rax)
 109:   0d 00 00 00 00          or     $0x0,%eax

from obj2asm:

00fe:   66 48 C7 06 2D 00       mov     qword ptr [RSI],02Dh
0104:   48 89 D9                mov     RCX,RBX
0107:   48 BA 0D 00 00 00 00 00 00 00   mov     RDX,0Dh
0111:   48 89 75 D0             mov     -030h[RBP],RSI

0x2d == '-'.  This is part of the buffer[$ - ndigits] = '-' line.