newCTFE Status July 2017

[Stefan Koch via Digitalmars-d]

On Thursday, 13 July 2017 at 12:45:19 UTC, Stefan Koch wrote:
> [ ... ]

I just figured out the bug in test/runnable/template8.d

What happens somewhere inside those templates is that the 
following expression is executed:

"some string literal" ~ null;

when then happens we executed the equivalent of the following in 
bytecode
{
   size_t lhsorrhs = lhs | rhs;
   if (!lhsorrhs)
     return null;
    // needed to handle the special case null ~ null == null;
    immutable elemSize = elementSize(lhs); // can be assumed to be 
the same as rhs
                                           // sema would have 
complained otherwise

   int newSize = 0;
   int lhsSize = lhs ? getLength(lhs) * elemSize : 0;
   int rhsSize = rhs ? getLength(rhs) * elemSize : 0;
   newSize += lhsSize;

   newSize += (getLength(rhs) * elemSize);

   void* newString = allocateHeap(newSize + 
SliceDescriptor.sizeof);
   auto sliceDesc = cast(SliceDescriptor*) newString;
   sliceDesc.base = newString + SliceDescriptor.sizeof;
   sliceDesc.length = newSize / elemSize;
   newString += SliceDescriptor.sizeof;

   memcpy(newString, lhs, lhsSize);
   memcpy(newString + lhsSize, rhs, rhsSize);

}

now what happens if either lhs OR rhs are null but not both ?
right a null pointer dereference.

and this is what happend here.

Why did it take so long to find ?

Well please scan the test
https://github.com/dlang/dmd/blob/master/test/runnable/template8.d
yourself and tell me where you see "something" ~ null :)