Well, I've made some progress:

• I still don't know what causes libtool to misbehave in parallel, but I've forced libphobos to be built with -j1, so that's at least a working workaround that doesn't slow down the rest of the build.
• The nixpkgs wrappers for gcc introduce the -idirafter flag, which is not accepted by gdc. It looks like this causes the check for the '-fPIC' flag to fail, and the flag to be dropped. This is the cause of the linker error for the shared libs. I've hacked around for a bit and got rid of the flag at the point of the check, so libphobos now builds correctly (but still with a bunch of warnings).
• I've added some nix code to generate a wrapper for gdc, like the ones that are generated for the rest of the GNU compiler family.

With these changes I can now build gdc + libraries and I've successfully built the example on the dlang.org front page from a nix shell.

Does anyone know of a nicer way to get rid of the -idirafter flag, that would help a lot in cleaning up this PR.

Maybe @ThomasMader has some pointers?

Rebased, seems to work, but it's still a bit ugly and I don't know how to prevent all -idirafter flags, or why libphobos doesn't want to be built in parallel.

I would like to see that merged into master.
GDC might be used to bootstrap LDC and for this alone it's very valuable.
I don't see any reasons why to hold this back for any longer if it is working. It can be improved over time but for now it's better to have something instead of nothing.

Can the eval errors be fixed?

Thanks a lot for your work @SFrijters.

I see there's merge conflicts again because this PR keeps lingering; I can probably look into it and rebase it again some time this weekend, but I would need some assistance to solve the failed evals (I think they were on platforms that are not available to me) and to get rid of the ugliness. I'm pretty much at the limit of my nix skills with this.

Edit: The failing checks I see now are different from the ones I remember. In this case I also have no idea what they mean, sad to say. Locally getting into a nix-shell works and I can still compile some randomly chosen example snippets from the D homepage.

I think I understand the previously failing checks now and I have rebased and fix them (I hope).
Let's see what new errors pop up. Still, locally I can build and use the compiler without apparent issues.

You are one commit behind. (8e79583#diff-a9fd46395fe31eaae37b3d89298f00cb)

Have you really rebased onto master?

I have, but I was unlucky and Ada has been added to gcc now too, so that was causing new merge conflicts. I think that was merged from staging to master since yesterday.

@worldofpeace This is also related to dlang. Can you also merge this one or is this special because it's gcc?

@worldofpeace This is also related to dlang. Can you also merge this one or is this special because it's gcc?

Yeah, this is special since it's a variant and we're touching cc-wrapper, which we should be careful with that. I'll see if @matthewbauer could maybe overlook it.

Rebased again, and built it using NixOS this time. I found I had to make another addition to the wrapper to make sure zlib is available when the compiler is invoked.

@worldofpeace The approvement by @matthewbauer was given now.

Rebased on master again to try and avoid merge conflicts later.

@Ericson2314 is there a way you could move this forward now that matthewbauer has approved this?

@matthewbauer Thanks!