Uncovered while doing a re-write for std.array.insertInPlace.
It got caught by pull auto-tester.

Sample:

import std.conv, std.utf;

void insertInPlace(T, U...)(ref T[] array, size_t pos, U stuff)
{
    {// mutable, can do in place
        //helper function: re-encode dchar to Ts and store at *ptr
        static T* putDChar(T* ptr, dchar ch)
        {
            static if(is(T == dchar))
            {
                *ptr++ = ch;
                return ptr;
            }
            else
            {
                T[dchar.sizeof/T.sizeof] buf;
                size_t len = encode(buf, ch);
                final switch(len)
                {
                static if(T.sizeof == char.sizeof)
                {
                case 4:
                    ptr[3] = buf[3];
                    goto case;
                case 3:
                    ptr[2] = buf[2];
                    goto case;
                }
                case 2:
                    ptr[1] = buf[1];
                    goto case;
                case 1:
                    ptr[0] = buf[0];
                }
                ptr += len;
                return ptr;
            }
        }
        
        immutable oldLen = array.length;
        size_t to_insert = 0;
 
        foreach (i, E; U)
            to_insert += codeLength!T(stuff[i]);
        array.length += to_insert;

        auto ptr = array.ptr + pos;
        foreach (i, E; U)
        {
            static if(is(E : dchar))
            {                
                ptr = putDChar(ptr, stuff[i]);
            }
            else
            {
                foreach (dchar ch; stuff[i])
                    ptr = putDChar(ptr, ch);
            }
        }
        assert(ptr == array.ptr + pos + to_insert, text(ptr - array.ptr, " vs ",pos+ to_insert ));
    }
}

void main()
{
	auto l = "hello"d.dup;
	auto r = " વિશ્વ".dup;
	l.insertInPlace(0, r);            	
}


Compiled as 
dmd -O atest.d

It fails with:
core.exception.AssertError@atest.d(75): 0 vs 6

Compiled without -O it doesn't fail. It also works fine in 32 bits.
Tested with dmd 2.061 from github. Commit a1287bd0b1931917f4e43b5e2d7b997f0d60adf6.