prevent clang-tidy warnings for extern libraries used

[ClausKlein]

i.e. the example use cxxopts which was fetched with CPM.cmake:

builddriver executing: 'run-clang-tidy.py -p build/standalone -checks=-*,modernize-*,misc-*,hicpp-*,cert-*,readability-*,portability-*,performance-*,google-* standalone'
Compilation SUCCEED in 8.628576 seconds
Number of warnings: 238
WarningErrorEntry(path='/Users/clausklein/.cache/CPM/cxxopts/d5c328043c71a6a5670c290d8646b093756db962/include/cxxopts.hpp', lineno='51', severity='warning', message="declaration uses identifier 'CXXOPTS__VERSION_MAJOR', which is a reserved identifier [cert-dcl37-c,cert-dcl51-cpp]", column='9')
WarningErrorEntry(path='/Users/clausklein/.cache/CPM/cxxopts/d5c328043c71a6a5670c290d8646b093756db962/include/cxxopts.hpp', lineno='52', severity='warning', message="declaration uses identifier 'CXXOPTS__VERSION_MINOR', which is a reserved identifier [cert-dcl37-c,cert-dcl51-cpp]", column='9')
WarningErrorEntry(path='/Users/clausklein/.cache/CPM/cxxopts/d5c328043c71a6a5670c290d8646b093756db962/include/cxxopts.hpp', lineno='53', severity='warning', message="declaration uses identifier 'CXXOPTS__VERSION_PATCH', which is a reserved identifier [cert-dcl37-c,cert-dcl51-cpp]", column='9')
WarningErrorEntry(path='/Users/clausklein/.cache/CPM/cxxopts/d5c328043c71a6a5670c290d8646b093756db962/include/cxxopts.hpp', lineno='212', severity='warning', message="use 'using' instead of 'typedef' [modernize-use-using]", column='3')
WarningErrorEntry(path='/Users/clausklein/.cache/CPM/cxxopts/d5c328043c71a6a5670c290d8646b093756db962/include/cxxopts.hpp', lineno='216', severity='warning', message='use a trailing return type for this function [modernize-use-trailing-return-type]', column='3')
WarningErrorEntry(path='/Users/clausklein/.cache/CPM/cxxopts/d5c328043c71a6a5670c290d8646b093756db962/include/cxxopts.hpp', lineno='223', severity='warning', message='use a trailing return type for this function [modernize-use-trailing-return-type]', column='3')
WarningErrorEntry(path='/Users/clausklein/.cache/CPM/cxxopts/d5c328043c71a6a5670c290d8646b093756db962/include/cxxopts.hpp', lineno='230', severity='warning', message='use a trailing return type for this function [modernize-use-trailing-return-type]', column='3')
WarningErrorEntry(path='/Users/clausklein/.cache/CPM/cxxopts/d5c328043c71a6a5670c290d8646b093756db962/include/cxxopts.hpp', lineno='230', severity='warning', message="non-const reference parameter 's', make it const or use a pointer [google-runtime-references]", column='23')
WarningErrorEntry(path='/Users/clausklein/.cache/CPM/cxxopts/d5c328043c71a6a5670c290d8646b093756db962/include/cxxopts.hpp', lineno='230', severity='warning', message="the parameter 'a' is copied for each invocation but only used as a const reference; consider making it a const reference [performance-unnecessary-value-param]", column='33')
WarningErrorEntry(path='/Users/clausklein/.cache/CPM/cxxopts/d5c328043c71a6a5670c290d8646b093756db962/include/cxxopts.hpp', lineno='232', severity='warning', message='passing result of std::move() as a const reference argument; no move will actually happen [hicpp-move-const-arg,performance-move-const-arg]', column='21')
# . . .

[ClausKlein]

Hint: all headers included with #include<...> and found with -isystem are suppressed by default (at least by gcc and clang compilers)

[ClausKlein]

When I change the generated compile_commands.json to:

[
{
  "directory": "/Users/clausklein/Workspace/cpp/ModernCppStarter/build/standalone",
  "command": "/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++ -DFMT_HEADER_ONLY=1 
-I/Users/clausklein/Workspace/cpp/ModernCppStarter/include -I_deps/greeter-build/PackageProjectInclude 
-I/Users/clausklein/.cache/CPM/fmt/9dd47b889fbf7c876d24be56ca097a884004b789/include 
-isystem /Users/clausklein/.cache/CPM/cxxopts/d5c328043c71a6a5670c290d8646b093756db962/include
-isystem /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1
-isystem /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/12.0.0/include 
-isystem /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.1.sdk/usr/include 
-isystem /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include  
-isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.1.sdk 
-std=c++17 -o CMakeFiles/GreeterStandalone.dir/source/main.cpp.o 
-c /Users/clausklein/Workspace/cpp/ModernCppStarter/standalone/source/main.cpp",
  "file": "/Users/clausklein/Workspace/cpp/ModernCppStarter/standalone/source/main.cpp"
}
]

there are no waring about cxxopts code 🥇

[ClausKlein]

This fix it temporarily:

perl -i.bak -pe 's#$CPM_SOURCE_CACHE-I#-isystem $1#g' build/install/compile_commands.json
run-clang-tidy.py -p build/install source
# . . .
# no warnings

[ClausKlein]

see too https://discourse.cmake.org/t/how-to-prevent-clang-tidy-to-check-sources-added-with-fetchcontents/2871