lsp mode does not recognize standard library

Question

Today I did two things:

• I upgraded some emacs packages (don't know which ones)
• I installed in addition to emacs 27.1, emacs 28 with native compilation

I built the "gccemacs" in a separate folder but started with the same config I use for emacs 27. But then I realized, lsp-mode (I am using it with ccls) does not recognize libraries like iostream or cstdint. So I wanted to use emacs 27.1 again but emacs 27.1 shows now the same behavior. Things like lsp-diagnose look fine. When I switch to a new project the echo area shows LSP :: CCLS:XXXX initialized successfully

Now when installing gccmacs I also installed gcc-9 and g++-9 and used the following commands:

sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-9 90 --slave /usr/bin/g++ g++ /usr/bin/g++-9 --slave /usr/bin/gcov gcov /usr/bin/gcov-9
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-7 70 --slave /usr/bin/g++ g++ /usr/bin/g++-7 --slave /usr/bin/gcov gcov /usr/bin/gcov-7

What weird is, is that I can use lsp-find-definition for libraries that get included via the compiler flags like e.g. armadillo library. If I use the command, I jump to /usr/include/armadillo but iostream or cmath can not be found. So this shows at least some minimal amount of functionality is there. Everything else looks fine, I don't get errors only a couple of warnings from other packages after the first byte compilations but that was the case before and it did work this morning. For some reason lsp-mode is not recognizing the standard library. Maybe I setup the new compiler version wrong? I switched back to gcc-7 anyways and gcc --version also tells that this is the case (same with g++). And yes, I can compile my programs and I use bear to execute a make file and generate the necessary .json file. Any ideas of what could be the cause of this issue?

EDIT1: Ok, some times when moving around in the buffer I get the echo message LSP :: Error from Language Server: not indexed (Invalid Request) I think this error also showed when it was working, but just in case.

EDIT2: I commented everything ccls related and used clangd instead but the issue persists. I then delete my .emacs.d folder and installed all packages from scratch with lsp-mode using only melpa-stable but the issue persists.

EDIT 3: I think I have come closer to a solution. It seems like lsp-mode or ccls/clangd does not recognize that I want to code c++ but c. I can include math.h but not cmath (without an error). Same behavior with iostream and stdio.h can I somehow tell lsp-mode to recognize c++?

EDIT 4: In the ccls faq is written, that the header file issue might occur if you e.g. have installed gcc-9 but not g++-9. But for me this is not the case. I have installed gcc-9, g++-9 and libstdc++-9 is installed as well. What I also realized is, that I can do: include <c++/7/iostream> without error because g++-7 is installed. Unfortunately, the ccls or lsp-mode does still complain about std:: (undeclared identifiert).

Answer 1

I solved the issue. It was not an emacs-ccls or lsp-mode issue but a clang issue. The issue started when I installed gcc-9 on Ubuntu-18.04 using the toolchain-r/test repo:

sudo add-apt-repository ppa:ubuntu-toolchain-r/test

I also noticed that I could not even compile a simple hello-world program using clang++. When compiling with the -v invocation it turned out that clang was looking for gcc-10 even though it was not installed:

[...]
Selected GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/10
[...]

Installing libstdc++-10 solved the issue.