This one is weird. I narrowed it down to this test case, although I admit this is still pretty long.

struct Test
{
    string s;
    
    this(string s)
    {
        this.s = s;
    }
    
    string ctfe()
    {
        dchar c = s[0];

        if (test1(c))
        {
            for (c = next(c); test2(c); c = next(c))
            {
            }
        }
        
        return "ctfe";
    }
    
    dchar next(dchar curPeek)
    {
        if (curPeek != dchar.init)
        {
            s = s[1..$];
            
            return s.length > 0 ? s[0] : dchar.init;
        }
        
        return dchar.init;
    }
}

bool test1(dchar c)
{
    return (c >= 'a' && c <= 'z' ||
            c >= 'A' && c <= 'Z' ||
            c == '_');
}

bool test2(dchar c)
{
    return (test1(c) ||
            c >= '0' && c <= '9' );
}

void main(string[] args)
{
    // If this is auto then this executes just fine at runtime
    enum x = Test("abc").ctfe();
}

This will ICE with an out of memory error. I believe it's going into an infinite loop.

If you replace the body of test2 with "return false", then CTFE runs just fine.

This only happens on 2.054. On 2.053 (when you make appropriate modifications to the test case) it crashes DMD.