feat(build): replace -Wc++-compat' with -x c++'

As of this commit, a build with CFLAGS="-x c++" succeeds; ie: all source
files in the tree are both valid C++ source code AND valid C code.

Convert wcpp-compat distcheck builds to actually build with C++, to
prevent future pull requests from breaking C++ compatibility going
forward.

stack-info: PR: aws#578, branch: aws-nslick/stack/25
Signed-off-by: Nicholas Sielicki <[email protected]>

.github/workflows/distcheck.yaml

@@ -103,6 +103,7 @@ jobs:
           ./configure --prefix=/opt/aws-ofi-nccl --with-mpi=/opt/amazon/openmpi \
                --with-libfabric=/opt/amazon/efa \
                --with-cuda=/usr/local/cuda \
+               --enable-cpp=${{ matrix.mode == 'cpp' && 'yes' || 'no' }} \
                --enable-tests=no \
                --enable-platform-aws
 
@@ -122,6 +123,9 @@ jobs:
         cc:
           - gcc
           - clang
+        mode:
+          - cpp
+          - c
         tracing:
           - lttng
           - none
@@ -137,7 +141,7 @@ jobs:
             cc: gcc
             cc-version: 13
 
-    name: u2204/${{ matrix.sdk }}/libfabric@git/${{matrix.cc}}(${{matrix.cc-variant}})/distcheck/
+    name: u2204/${{ matrix.sdk }}/libfabric@git/${{matrix.cc}}(${{matrix.cc-variant}})/distcheck(${{ matrix.mode }})/
     steps:
       - uses: actions/checkout@v4
       - name: Configure Neuron SDK Repository
@@ -219,10 +223,12 @@ jobs:
             ./configure --with-mpi=/opt/amazon/openmpi \
                         --with-libfabric=/opt/amazon/efa \
                         --with-cuda=/usr/local/cuda/ \
+                        --enable-cpp=${{ matrix.mode == 'cpp' && 'yes' || 'no' }} \
                         --enable-tests \
                         --enable-platform-aws
           else
             ./configure --with-libfabric=/opt/amazon/efa \
+                        --enable-cpp=${{ matrix.mode == 'cpp' && 'yes' || 'no' }} \
                         --enable-neuron \
                         --enable-platform-aws
           fi
@@ -249,6 +255,9 @@ jobs:
         cc:
           - gcc
           - clang
+        mode:
+          - cpp
+          - c
         sdk:
           - cuda
           - neuron
@@ -260,7 +269,7 @@ jobs:
             cc: gcc
             cc-version: 13
 
-    name: u2204/${{ matrix.sdk }}/${{matrix.cc}}(${{matrix.cc-variant}})/unit-tests/
+    name: u2204/${{ matrix.sdk }}/${{matrix.cc}}(${{matrix.cc-variant}})/unit-tests(${{ matrix.mode }})/
     steps:
       - uses: actions/checkout@v4
       - name: Configure Neuron SDK Repository
@@ -337,13 +346,15 @@ jobs:
             ./configure --with-mpi=/opt/amazon/openmpi \
                         --with-libfabric=/opt/amazon/efa \
                         --with-cuda=/usr/local/cuda/ \
+                        --enable-cpp=${{ matrix.mode == 'cpp' && 'yes' || 'no' }} \
                         --enable-tests \
                         --enable-debug \
                         --enable-platform-aws
           else
             ./configure --with-libfabric=/opt/amazon/efa \
                         --enable-neuron \
                         --enable-debug \
+                        --enable-cpp=${{ matrix.mode == 'cpp' && 'yes' || 'no' }} \
                         --enable-platform-aws
           fi
           make -j "$(nproc)"
@@ -367,7 +378,10 @@ jobs:
         sdk:
           - cuda
           - neuron
-    name: CodeChecker - ${{ matrix.sdk }}
+        mode:
+          - cpp
+          - c
+    name: CodeChecker - ${{ matrix.sdk }} ${{ matrix.mode }}
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v5
@@ -423,12 +437,14 @@ jobs:
             ./configure \
               --with-libfabric="/opt/amazon/efa" \
               --enable-neuron \
+              --enable-cpp=${{ matrix.mode == 'cpp' && 'yes' || 'no' }} \
               --enable-platform-aws
           else
             ./configure \
               --with-libfabric="/opt/amazon/efa" \
               --with-mpi="/opt/amazon/openmpi" \
               --with-cuda=/usr/local/cuda/ \
+              --enable-cpp=${{ matrix.mode == 'cpp' && 'yes' || 'no' }} \
               --enable-tests \
               --enable-platform-aws
           fi
@@ -447,7 +463,7 @@ jobs:
       - name: Save CodeChecker HTML output.
         uses: actions/upload-artifact@v4
         with:
-          name: CodeChecker Bug Reports for ${{ matrix.sdk }}
+          name: CodeChecker Bug Reports for ${{ matrix.sdk }} ${{ matrix.mode }}
           path: ${{ steps.codechecker.outputs.result-html-dir }}/*.html
 
       - name: CodeChecker Pass Or Fail?

configure.ac

@@ -173,9 +173,6 @@ AC_DEFINE_UNQUOTED([OFI_NCCL_TRACE], [${trace}], [Defined to 1 unit test output
 
 dnl standard and normal
 picky_compiler_flags="-Wall -Wextra"
-dnl C specific warning that warngs when using C features which prevent parsing
-dnl as C++. Best-effort, and doesn't cover everything.
-picky_compiler_flags="${picky_compiler_flags} -Wc++-compat"
 dnl some top-level API functions have unused parameters, but we have to keep
 dnl them. TODO: explicitly annotate those functions, rather than setting this
 dnl globally.
@@ -215,6 +212,12 @@ AS_IF([test -d "${srcdir}/.git" -a -z "${enable_werror}"],
 
 AC_SUBST([NCCL_NET_OFI_DISTCHCK_CONFIGURE_FLAGS])
 
+AC_ARG_ENABLE([cpp],
+   [AS_HELP_STRING([--enable-cpp], [(Developer) Enable building as C++ source. Off by default.])])
+AS_IF([test "${enable_cpp}" = "yes"],
+      [AC_MSG_NOTICE([Building as C++ source.])
+       CFLAGS="${CFLAGS} -x c++ -std=c++17"])
+
 AC_CONFIG_FILES([Makefile
                  include/Makefile
                  3rd-party/Makefile