Reason for missing notes for incomplete-type errors in standard headers

[Julian Lenz <j.j.lenz@swansea.ac.uk>]

Hi everybody,

TL;DR:

What is the reason that `error: '<some class>' has incomplete type` does 
not give a note about where the forward declaration happened for 
standard library classes?

Full:

Recently, I forgot to include <array> but had <tuple> included. Due to 
this recent commit for gcc12

https://github.com/gcc-mirror/gcc/commit/261d5a4a459bd49942e53bc83334ccc7154a09d5

the `error: 'std::array<int,N>' has incomplete type` felt like a weird 
regression because all other major gcc versions as well as clang 
compiled just fine. After many hours of debugging, I finally realised 
that it was just a missing header and now the error message actually 
makes sense.

But when I experimented with this, I found that gcc does give you 
another `<filename>:<line number>:<column number>: note: declaration of 
'class SomeClass<int>'` with line number and everything. Now, I wonder 
why gcc doesn't do that for standard headers (at least not in my 
particular case). Is there a compelling argument for that? It feels like 
it must be intentional. If not, I would suggest adding that because I'm 
pretty sure it would have spared me a couple of hours just by realising 
quickly that it takes `std::array` from the wrong header.

Thank you in advance and keep up the great work!

Julian