Recent clang with C++23 enabled gives warnings when compiling IDL files

[jwillemsen]

Recent clang versions with C++23 enabled gives warnings when compiling IDL files because clang recognizes module as C++ keyword and thinks we are using C++ modules.

Warning TAO_Ext.pidl 31(10): '#include <filename>' attaches the declarations to the named module 'CORBAlocalinterfaceCurrent', which is not usually intended; consider moving that directive before the module declaration [-Winclude-angled-in-module-purview]
   31 | #include <tao/TimeBase.pidl>
      |          ^

[iguessthislldo]

The first thing that came to my mind is to replace module with something like s/\bmodule\b/__IDL_MODULE/g before the preprocessor, then swap it back (s/\b__IDL_MODULE\b/module/g) or maybe defining the replacement as a macro (-D__IDL_MODULE=module) might be enough to trick it . Could also insert a pragma to disable the warning into the preprocessor input. Either way that would be unfortunate though because the point of #1357 was to stop using temporary files as the preprocessor input.

[jwillemsen]

Or pass some additional arguments when invoking the compiler as preprocessor, maybe just set c++17 as c++ level

[mitza-oci]

This warning is from clang preprocessing the IDL? If you run tao_idl with -v what's the command line used to launch the preprocessor?

[mitza-oci]

Since one of our scoreboard builds here https://www.dre.vanderbilt.edu/scoreboard/tao.html uses Clang 20 (latest release), I tried this out. I don't see the same warning.

/var/lib/jenkins/workspace/ACE_TAO_Ubuntu_22_04_Clang20_x64_Debug/ACE/bin/tao_idl: preprocessing TAO_Ext.pidl
/var/lib/jenkins/workspace/ACE_TAO_Ubuntu_22_04_Clang20_x64_Debug/ACE/bin/tao_idl: spawning: clang++-20 -D__TAO_IDL=0x080004 -I. -I/var/lib/jenkins/workspace/ACE_TAO_Ubuntu_22_04_Clang20_x64_Debug/TAO -I/var/lib/jenkins/workspace/ACE_TAO_Ubuntu_22_04_Clang20_x64_Debug/TAO/tao -E -I../.. -DGEN_IDL_MAKE_DEPS -D__TAO_IDL_IDL_VERSION=0x030000 -D__TAO_IDL_FEATURES="tao/idl_features.h" -I/var/lib/jenkins/workspace/ACE_TAO_Ubuntu_22_04_Clang20_x64_Debug/TAO/orbsvcs /tmp/tao-idli_eL6uV2.cpp
/var/lib/jenkins/workspace/ACE_TAO_Ubuntu_22_04_Clang20_x64_Debug/ACE/bin/tao_idl: parsing TAO_Ext.pidl
processing TAO_Ext.pidl
/var/lib/jenkins/workspace/ACE_TAO_Ubuntu_22_04_Clang20_x64_Debug/ACE/bin/tao_idl: BE processing on TAO_Ext.pidl

[jwillemsen]

Yes, the warning is from the preprocessing of the IDL file. This is a custom compiler using clang, it uses C++23 as default. I will try to make c++17 the default using some defines in TAO_IDL.

[mitza-oci]

I didn't know that the definition of preprocessing tokens changed in C++20 -- I don't think any of the most commonly-used compilers are defaulting to 20 yet, but it's good to get ahead of it.

https://en.cppreference.com/w/cpp/language/translation_phases.html#Preprocessing_tokens

[jwillemsen]

Also of impact for ACE/TAO is that with C++23 the libc++ headers change, potentially causing issues, see https://libcxx.llvm.org/DesignDocs/HeaderRemovalPolicy.html

[jwillemsen]

#2421