[cmake] Enable Clang warnings about redundant semicolons

[mstorsjo]

Summary

This matches what GCC warns about when -pedantic is enabled.

This should avoid such redundant semicolons creeping into the codebase.

[xbolva00]

Shouldn’t we rather add that warning to -pedantic in Clang as well?

cc @aaron.ballman @rsmith

[mstorsjo]

In D98941#2637120, @xbolva00 wrote:

Shouldn’t we rather add that warning to -pedantic in Clang as well?

cc @aaron.ballman @rsmith

Maybe; Clang does warn about it if building with -std=c++98, in the form warning: extra ';' outside of a function is a C++11 extension [-Wc++11-extra-semi] though. But in C++11 mode, it only warns about it if this option is added, warning: extra ';' outside of a function is incompatible with C++98 [-Wc++98-compat-extra-semi]. I'd say the Clang behaviour in itself makes sense (it's a new feature and you're supposed to be able to use it), so GCC is the odd one out there.

[aaron.ballman]

In D98941#2637120, @xbolva00 wrote:

Shouldn’t we rather add that warning to -pedantic in Clang as well?

cc @aaron.ballman @rsmith

I think GCC's behavior is not particularly helpful -- pedantically, the code is correct (in C++11 and later mode).

LGTM with a small tweak!

llvm/cmake/modules/HandleLLVMOptions.cmake

675

[mstorsjo]

llvm/cmake/modules/HandleLLVMOptions.cmake

675

Thanks! Will fix that before pushing.

[nickdesaulniers]

I think one of the buildbots is failing with this IIUC?
https://lab.llvm.org/buildbot/#/builders/4/builds/8255
/b/1/openmp-gcc-x86_64-linux-debian/llvm.src/openmp/runtime/src/kmp_i18n.h:173:2: warning: extra ';' outside of a function is incompatible with C++98 [-Wc++98-compat-extra-semi]

Looks like a straightforward fix, though that code hasn't changed recently, and this change went in over a month ago.

[mstorsjo]

In D98941#2730846, @nickdesaulniers wrote:

I think one of the buildbots is failing with this IIUC?
https://lab.llvm.org/buildbot/#/builders/4/builds/8255
/b/1/openmp-gcc-x86_64-linux-debian/llvm.src/openmp/runtime/src/kmp_i18n.h:173:2: warning: extra ';' outside of a function is incompatible with C++98 [-Wc++98-compat-extra-semi]

Looks like a straightforward fix, though that code hasn't changed recently, and this change went in over a month ago.

It doesn't seem like it's failing over that to me? But it does indeed seem to spam a number of warnings in that setup. I pushed a commit to get rid of those redundant semicolons at least, in rG01d27fc40836 - thanks for noticing!

llvm/cmake/modules/HandleLLVMOptions.cmake

if (CXX_SUPPORTS_MISSING_FIELD_INITIALIZERS_FLAG)
     else()
       append("-Wmissing-field-initializers" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
     endif()
   endif()
 
   if (LLVM_ENABLE_PEDANTIC AND LLVM_COMPILER_IS_GCC_COMPATIBLE)
     append("-pedantic" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
     append("-Wno-long-long" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
+
+    # GCC warns about redundant toplevel semicolons (enabled by -pedantic
+    # above), while Clang doesn't. Enable the corresponding Clang option to
+    # pick up on these even in builds with Clang.
+    add_flag_if_supported("-Wc++98-compat-extra-semi" CXX98_COMPAT_EXTRA_SEMI_FLAG)
   endif()
 
   add_flag_if_supported("-Wimplicit-fallthrough" IMPLICIT_FALLTHROUGH_FLAG)
   add_flag_if_supported("-Wcovered-switch-default" COVERED_SWITCH_DEFAULT_FLAG)
   append_if(USE_NO_UNINITIALIZED "-Wno-uninitialized" CMAKE_CXX_FLAGS)
   append_if(USE_NO_MAYBE_UNINITIALIZED "-Wno-maybe-uninitialized" CMAKE_CXX_FLAGS)
 
   # Disable -Wclass-memaccess, a C++-only warning from GCC 8 that fires on