Comment # 3 on bug 1199943 from Andreas Nordal

clang++: Same story, as expected (I only used options in compile_commands.json
that both compilers support).

I found a way to eliminate the editor from the equation:
clangd --check=takes_string_view.cpp

Output of that on openSUSE Leap 15.4:

I[19:42:18.098] clangd version 13.0.1
I[19:42:18.099] Features: linux
I[19:42:18.099] PID: 19436
I[19:42:18.099] Working directory: /tmp
I[19:42:18.099] argv[0]: clangd
I[19:42:18.099] argv[1]: --log=info
I[19:42:18.099] argv[2]: --check=takes_string_view.cpp
I[19:42:18.099] Entering check mode (no LSP server)
I[19:42:18.099] Testing on source file /tmp/takes_string_view.cpp
I[19:42:18.099] Loading compilation database...
I[19:42:18.099] Loaded compilation database from /tmp/compile_commands.json
I[19:42:18.100] Compile command from CDB is: /usr/bin/clang++ --driver-mode=g++
-Iweird_include -c -resource-dir=/usr/lib64/clang/13.0.1 --
/tmp/takes_string_view.cpp
I[19:42:18.100] Parsing command...
I[19:42:18.101] internal (cc1) args are: -cc1 -triple x86_64-suse-linux-gnu
-fsyntax-only -disable-free -disable-llvm-verifier -discard-value-names
-main-file-name takes_string_view.cpp -mrelocation-model static
-mframe-pointer=all -fmath-errno -fno-rounding-math -mconstructor-aliases
-munwind-tables -target-cpu x86-64 -tune-cpu generic -debugger-tuning=gdb
-fcoverage-compilation-dir=/tmp -resource-dir /usr/lib64/clang/13.0.1 -I
weird_include -internal-isystem
/usr/bin/../lib64/gcc/x86_64-suse-linux/7/../../../../include/c++/7
-internal-isystem
/usr/bin/../lib64/gcc/x86_64-suse-linux/7/../../../../include/c++/7/x86_64-suse-linux
-internal-isystem
/usr/bin/../lib64/gcc/x86_64-suse-linux/7/../../../../include/c++/7/backward
-internal-isystem /usr/lib64/clang/13.0.1/include -internal-isystem
/usr/local/include -internal-isystem
/usr/bin/../lib64/gcc/x86_64-suse-linux/7/../../../../x86_64-suse-linux/include
-internal-externc-isystem /include -internal-externc-isystem /usr/include
-fdeprecated-macro -fdebug-compilation-dir=/tmp -ferror-limit 19
-fgnuc-version=4.2.1 -fcxx-exceptions -fexceptions -faddrsig
-D__GCC_HAVE_DWARF2_CFI_ASM=1 -x c++ /tmp/takes_string_view.cpp
I[19:42:18.101] Building preamble...
I[19:42:18.250] Indexing headers...
I[19:42:18.325] Building AST...
E[19:42:18.344] [typename_nested_not_found] Line 7: no type named 'string_view'
in namespace 'std'
I[19:42:18.344] Indexing AST...
I[19:42:18.345] Testing features at each token (may be slow in large files)
I[19:42:18.347] All checks completed, 1 errors

--log=info is apparently the default (gives the same output).
The verbose log is more verbose, but doesn't add more clues that I can see.

What I see is that -std=c++17 looks suspiciously absent in those internal args.
Indeed, I can reproduce the error by running clang++ with those arguments,
and then make it go away by adding -std=c++17!

I think I found the missing argument! Dunno how to fix it though.