Need help locating AA bug

[Jonathan M Davis]

On Monday, February 27, 2012 17:28:17 H. S. Teoh wrote:
> On Mon, Feb 27, 2012 at 02:41:27PM -0800, H. S. Teoh wrote:
> > I'm investigating issue 7512, and narrowed down the problem to a bug
> > somewhere in the code that handles AA literals.
> 
> [...]
> 
> OK, I've found the cause of the problem. Suppose you have an int array:
> 
> const int[] key = [1,2,3,4];
> 
> and you create an AA out of it with an AA literal:
> 
> int[int[]] map1 = [ key: 1234 ];
> 
> then internally, map1's key TypeInfo points to const(int)[].
> 
> However, if you do this:
> 
> int[int[]] map2;
> map2[key] = 1234;
> 
> then internally, map2's key TypeInfo points to int[]. (Non-const!)
> 
> This difference means that the hash value of 'key' in map1 is
> *different* from the hash value of 'key' in map2. But if you now write
> this:
> 
> map1[key] = 1234;
> 
> for some reason, the hash value of 'key' is *still* computed using
> int[], even though map1's key TypeInfo points to const(int)[]. So the
> wrong hash value is computed, and the entry is not found. This means
> that you will never find anything in map1 using map1.get or map1[...].
> But foreach works 'cos no hash values are involved.
> 
> The above line will actually create a duplicate entry in the hash table,
> because 'key' is hashed to a different value than the one set by the AA
> literal.
> 
> So the question is:
> 
> 1) Why does map2's internal TypeInfo get set to int[] instead of
> const(int)[]?
> 
> 2) Why does map1[key] use int[]'s version of getHash() even though
> map1's TypeInfo actually points to const(int)[]?
> 
> 
> This completely breaks AA literals for wstring keys, dstring keys, and
> in fact *any* array-typed key except for string (which for some blessed
> reason doesn't suffer from this breakage).
> 
> Question: is this a compiler bug, or a druntime bug?

The keys to AA's are supposed to be _immutable_. Really, this should be 
enforced by the compiler (and it is on some level, but not to the point that 
code like int[int[]] is illegal like it should be). The fact that the 
implementation is using const internally just makes it that much worse.

- Jonathan M Davis