Compiler Bug using D interface file for its intended purpose

[Rambutan]

On Tuesday, 8 September 2026 at 09:43:22 UTC, Iain Buclaw wrote:
>
> Note: D default initializes floating point and character types 
> to a non-zero mask, which necessitates the need for an `.init` 
> symbol.
>
> To be compatible with C, use `= void` or `= 0`.
>
> ```d
> struct b_real {
>     double val = void;
> }
> ```

Ah, I should have put that in my example, it *is* everywhere in 
the real .di file in the big project I mentioned. That's why I 
was able to say this problem only exists when there's a double in 
a struct.

>> $ gdc -o breal breal.o dmain.o cmain.o #di.o
>> /usr/bin/ld: breal.o:(.data.rel.ro+0x28):
>>  undefined reference to `initializer for di.b_real'

I should have noticed the above missing initializer in my toy 
example did not appear as a linking issue in the real world 
version I'm working on, which it would presumably have done for 
*every* struct without there being `= void` everywhere.

> For the xtoHash and xopEquals functions, don't shy away from 
> raising a discussion against DMD - defer creating an issue till 
> later.
>
> https://github.com/dlang/dmd/discussions

OK, I will look into this.

> Without looking (or remembering) I suspect floating point 
> numbers might have non-trivial hashing requirement for edge 
> cases such as comparing `+0.0` and `-0,0`.

Ah, makes sense, and because of this sort of thing floating point 
equality is more than bitwise equality. So this explains the need 
for both xtoHash and xopEquals.

> If it isn't publicly documented, it ought to be mentioned 
> somewhere.
>
> But it is also true to say that if you are `new`ing the struct 
> on the GC, then runtime TypeInfo will *always* be pulled in 
> creating these sorts of dependencies.

Yes, I see that. So a .di file needs to be carefully constructed 
if it is to be able to evade needing to be compiled as D and 
linked in, and right now that cannot be arranged for a struct 
containing a double.

I found that compiling and linking in di.di in the above example 
did not resolve the missing xtoHash and xopEquals dependencies. I 
had to rename it to di.d to do that.

Thank you for the detailed reply.
R.