How to use ALE with clangd and C++20

Question

I want to write code like this in vim (vim 9.0)

#include <iostream>
#include <string>

template <class T1, class T2>
concept composable = requires(T1 lhs, T2 rhs) { lhs + rhs; };

template <class T1, class T2>
requires composable<T1, T2>
[[nodiscard]]
constexpr auto compose(T1 lhs, T2 rhs) -> decltype(lhs + rhs)
{
    return lhs + rhs;
}

auto main() -> int
{
    using std::cout, std::endl, std::string;
    cout << compose(5, 8) << endl;
    cout << compose(5, 8.4) << endl;
    cout << compose(string("Hello "), string("world!")) << endl;

    return 0;
}

However when I do, I get lots of error messages by the language server, like

compose.cpp|6 col 1 error| unknown type name 'concept' [clang-diagnostic-error]
compose.cpp|6 col 31 error| 'T1' does not refer to a value [clang-diagnostic-error]
compose.cpp|6 col 39 error| 'T2' does not refer to a value [clang-diagnostic-error]
compose.cpp|11 col 1 error| unknown type name 'requires' [clang-diagnostic-error]
compose.cpp|11 col 10 error| variable template partial specialization does not specialize any template argument; to define the primary template, remove the template argument list [clang-diagnostic-error]
compose.cpp|12 col 14 error| expected ';' at end of declaration [clang-diagnostic-error]
compose.cpp|13 col 24 error| unknown type name 'T1' [clang-diagnostic-error]
compose.cpp|13 col 32 error| unknown type name 'T2' [clang-diagnostic-error]

Which suggests that c++20 is not being used. However when I compile that code, it works flawlessly with both g++ and clang++ with no errors whatsoever.

I use dense-analysis/ale. I have read in numerous places, like this post, including the clangd documentation, that I should either create a file in ~/.config/clangd/config.yaml for a global user configuration, or a ./.clangd file for a project configuration.

I have done both, but neither seems to have any effect whatsoever.

First I created the user configuration, because I don't like littering my project with random config files and like to be consistent between my projects anyway. That file is in /home/user/.config/clangd/config.yaml and looks like so:

CompileFlags:
  Add: [-std=c++20, -Wall, -Wextra, -Wpedantic, -Wdouble-promotion, -Wformat=2, -Wformat-nonliteral, -Wformat-y2k, -Wnull-dereference, -Wimplicit-fallthrough, -Wmissing-include-dirs, -Wswitch-default, -Wunused-parameter, -Wuninitialized, -Wsuggest-attribute=const, -Walloc-zero, -Walloca, -Wconversion, -Wfloat-conversion, -Wsign-conversion, -Wduplicated-branches, -Wduplicated-cond, -Wtrampolines, -Wfloat-equal, -Wshadow=compatible-local, -Wundef, -Wunused-macros, -Wcast-qual, -Wcast-align=strict, -Wlogical-op, -Wmissing-declarations, -Wredundant-decls, -Wstack-protector, -fstack-protector, -pedantic-errors, -Werror=pedantic, -Werror=char-subscripts, -Werror=null-dereference, -Werror=init-self, -Werror=implicit-fallthrough=2, -Werror=misleading-indentation, -Werror=missing-braces, -Werror=multistatement-macros, -Werror=sequence-point, -Werror=return-type, -Werror=multichar,-Wno-unknown-warning-option]

That had no effect on the error messages being displayed in vim. So I created the .clangd file in my project. The contents are

-std=c++20

This also didn't work at all. What am I doing wrong? How do I get to write code with c++20 features and not get false errors?

I also tried

CompileFlags:
  Add: [-std=c++20]

with the same results.

I also tried setting

let g:ale_cpp_cc_options = '-std=c++20 -fstack-protector -Wall -Wextra -Wpedantic -Wdouble-promotion -Wformat=2 -Wformat-nonliteral -Wformat-signedness -Wformat-y2k -Wnull-dereference -Wimplicit-fallthrough=2 -Wmissing-include-dirs -Wswitch-default -Wunused-parameter -Wuninitialized -Wsuggest-attribute=const -Walloc-zero -Walloca -Wconversion -Wfloat-conversion -Wsign-conversion -Wduplicated-branches -Wduplicated-cond -Wtrampolines -Wfloat-equal -Wshadow=compatible-local -Wundef -Wunused-macros -Wcast-qual -Wcast-align=strict -Wlogical-op -Wmissing-declarations -Wredundant-decls -Wstack-protector -pedantic-errors -Werror=pedantic -Werror=char-subscripts -Werror=null-dereference -Werror=init-self -Werror=implicit-fallthrough=2 -Werror=misleading-indentation -Werror=missing-braces -Werror=multistatement-macros -Werror=sequence-point -Werror=return-type -Werror=multichar -Wno-unknown-warning-option'

in my .vimrc, which also didn't get rid of the false errors.

Clang and clang tools, including clangd and clang-tidy versions are Ubuntu clang version 17.0.0 (++20230623042322+7175d6a5966a-1~exp1~20230623042442.1015), but again, actually compiling the source code with g++ and clang yields no errors or warnings, as long as I specify -std=c++20.

EDIT
I switched to coc and that works just fine.

Answer 1

I am also using Vim 9.0 with ALE plugin

I am also configuring ALE for different programming languages. I saw your post and decided to try C++ first. Thank you for posting this query.

I tried your code, ALE worked as expected. I only used .vimrc file to define everything (no other files)

" CPP-SETTINGS:
" Lint .h files as C++, not C
let g:ale_pattern_options_enabled = 1
let g:ale_pattern_options = { '\.h$': { 'ale_linters': { 'cpp' : ['cc', 'gcc', 'clang'] } } }

" Set flags for gcc/clang
let cpp_opts = '-std=c++20 -Wall -Wextra'
let g:ale_linters = { 'cpp': ['cc', 'gcc', 'clang'] }
let g:ale_cpp_cc_options    = cpp_opts
let g:ale_cpp_gcc_options   = cpp_opts
let g:ale_cpp_clang_options = cpp_opts