Eigen 5.0.1 leads to massive number of compiler warnings

[WardBrian]

After #3271 was merged, running make examples/bernoulli/bernoulli up in CmdStan with gcc 13.3 yields 2991 lines of output from 281 warnings.
Almost all of them are Wmaybe-uninitialized stemming from the file Eigen/src/Core/arch/AVX512/TrsmUnrolls.inc

I am not sure how best to proceed, it seems like it might be a good idea to have Eigen under -isystem rather than -I, but we've historically avoided this

cc @SteveBronder

[SteveBronder]

In the short term we can add -Wno-maybe-uninitialized. We can then patch it locally. I'm also doing the patch for RcppEigen and can do a patch for Eigen as well

[syclik]

The reason we can't use `-isystem` is that the different compilers have different behaviors when the includes are included through `-isystem` vs `-I`. It's caused some major headaches in the past; I think in the most common failure mode, it will move the include order to behind other copies of libraries (so system Eigen) which cause other compiler issues. That said, try again. (I'm sure I wrote it out before and we can find it.)

…

On Mon, Jun 15, 2026 at 11:09 AM Steve Bronder ***@***.***> wrote: *SteveBronder* left a comment (stan-dev/math#3331) <#3331 (comment)> In the short term we can add -Wno-maybe-uninitialized. We can then patch it locally. I'm also doing the patch for RcppEigen and can do a patch for Eigen as well — Reply to this email directly, view it on GitHub <#3331?email_source=notifications&email_token=AADH6F4FZBAWPC3ONMQO2Y35AAGT7A5CNFSNUABFM5UWIORPF5TWS5BNNB2WEL2JONZXKZKDN5WW2ZLOOQXTINZQHEZTMMZVGI32M4TFMFZW63VKON2WE43DOJUWEZLEUVSXMZLOOSWGM33PORSXEX3DNRUWG2Y#issuecomment-4709363527>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AADH6FZFT3IPZCEITJYTWQD5AAGT7AVCNFSNUABEKJSXA33TNF2G64TZHMZTQMZYHA2DIMB3JFZXG5LFHM2DMNRWGIYTCNJYGCQXMAQ> . You are receiving this because you are subscribed to this thread.Message ID: ***@***.***>