[clang] [libcxx] [llvm] Add unnecessary-virtual-specifier to -Wextra (PR #138741)

[Martin Storsjö via cfe-commits]

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@@ -882,6 +882,11 @@ if (LLVM_ENABLE_WARNINGS AND (LLVM_COMPILER_IS_GCC_COMPATIBLE OR CLANG_CL))
   # The LLVM libraries have no stable C++ API, so -Wnoexcept-type is not useful.
   append("-Wno-noexcept-type" CMAKE_CXX_FLAGS)
 
+  # LLVM has a policy of including virtual "anchor" functions to control
+  # where the vtable is emitted. In `final` classes, these are exactly what
+  # this warning detects: unnecessary virtual methods.
+  add_flag_if_supported("-Wno-unnecessary-virtual-specifier" CXX_SUPPORTS_UNNECESSARY_VIRTUAL_FLAG)
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mstorsjo wrote:

Ok, thanks, then I'll just wait for that one to settle instead of investing more time into this right now.

https://github.com/llvm/llvm-project/pull/138741