[dmd-internals] Mixin forward reference semantic runs and field declarations

[Walter Bright]

That looks like a compiler bug.

On 8/30/2012 4:00 PM, David Nadlinger wrote:
> I am currently working on ironing out the last few kinks in the LDC
> merge of the 2.060 frontend. The following snippet, a test case for
> Bugzilla issue 7974, currently doesn't work with LDC:
>
> ---
> mixin template mix7974() {
>    uint _x;
> }
>
> struct Foo7974 {
>    immutable fa = Foo7974(0);
>
>    this(uint x) {
>      _x = x;
>    }
>    mixin mix7974!();
> }
> ---
>
> The problem is that the fix Walter applied to the frontend for 7974
> causes the semantic pass, including the arrayCopy() of the mixin
> contents into the structs, to run twice, which leads to two separate
> VarDeclarations for Foo7974._x being present at codegen time: one
> instance, the one from the »forward reference« semantic run, is
> referred to by the struct constructor, the other instance from the
> second, »proper« run is e.g. what ends up in
> StructDeclaration::fields.
>
> As we tuck on IR construction metadata to field declarations in LDC,
> having two instances for the same field breaks our codegen in
> interesting ways – is this an unforeseen consequence of the fix in DMD
> and  possibly a bug, or do I have to find a way to work around this in
> LDC?
>
> Thanks,
> David
>
>