ldc -O leads to wrong results

[Dude via digitalmars-d-ldc]

On Tuesday, 10 March 2015 at 05:35:59 UTC, Dan Olson wrote:
> "salsa" <salsa at salsa.com> writes:
>
>> Recently I got stuck with this:
>> compiling following code with ldc -O generates code that 
>> doesn't do
>> what I expect it to do. Compiling it without optimizations 
>> works
>> fine. Does somebody have any idea what the problem might be? To
>> reproduce the bug just compile and run once with 'ldc *.d' and 
>> once
>> with 'ldc -O *.d'...
>>
>> 			x[ 4] ^= rotl((x[ 0]+x[12]), 7);
> ...
>> 	final static T rotl(T)(T x, T y) pure nothrow @nogc
>> 	{
>> 		return (x << y) | (x >>> -y);
>> 	}
>
> Hi salsa.
>
> y is 7 so (x >>> -7) yields an unspecified value because shift 
> is
> negative (see TDPL 2.3.10 Shift Expression).  I think results 
> are only
> defined when shifting 0 to nbits-1.

Maybe this should be explicitly said in documentation:
http://dlang.org/expression.html#ShiftExpression

Btw. why this is not compile error on the first place?