[clang] [llvm] Enable unnecessary-virtual-specifier by default (PR #133265)

[Hans Wennborg via llvm-commits]

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@@ -689,7 +689,7 @@ if ( LLVM_COMPILER_IS_GCC_COMPATIBLE OR CMAKE_CXX_COMPILER_ID MATCHES "XL" )
 endif( LLVM_COMPILER_IS_GCC_COMPATIBLE OR CMAKE_CXX_COMPILER_ID MATCHES "XL" )
 
 if (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
-  append("-Werror=unguarded-availability-new" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
+  append("-Werror=unguarded-availability-new -Wno-unnecessary-virtual-specifier" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
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zmodem wrote:

I think the flag deserves a line of its own, along with a comment which explains that this is due to llvm's pattern of using anchor functions, which are exactly what the warning is targeting: unnecessary virtual functions.

I also wonder if we can pass the flag unconditionally like this, or if it will trigger `-Wunknown-warning-option` when building with an older clang. Maybe `-Wunknown-warning-option` is not enabled in llvm builds, in which case we're fine, otherwise maybe we need to do a version check here or something (see e.g. -Wno-class-memaccess or -Wc++98-compat-extra-semi in this file for inspiration).

https://github.com/llvm/llvm-project/pull/133265