[libc++] Enable -Wweak-vtables

[philnik]

Summary

This makes Clang scream at us if there is a class without a key function.

[ldionne]

I think this is a good idea, except for the number of diagnostics it generates that we don't really care about (in the test suite).

[philnik]

• Enable the warning only for clang-tidy

libcxx/include/__functional/function.h

 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 // bad_function_call
 
+_LIBCPP_DIAGNOSTIC_PUSH
+_LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wweak-vtables")
 class _LIBCPP_EXCEPTION_ABI bad_function_call
     : public exception
 {
 public:
 // Note that when a key function is not used, every translation unit that uses
 // bad_function_call will end up containing a weak definition of the vtable and
 // typeinfo.
 #ifdef _LIBCPP_ABI_BAD_FUNCTION_CALL_KEY_FUNCTION
     virtual ~bad_function_call() _NOEXCEPT;
 #else
     virtual ~bad_function_call() _NOEXCEPT {}
 #endif
 
 #ifdef _LIBCPP_ABI_BAD_FUNCTION_CALL_GOOD_WHAT_MESSAGE
     virtual const char* what() const _NOEXCEPT;
 #endif
 };
+_LIBCPP_DIAGNOSTIC_POP
 
 _LIBCPP_NORETURN inline _LIBCPP_INLINE_VISIBILITY
 void __throw_bad_function_call()
 {
 #ifndef _LIBCPP_NO_EXCEPTIONS
     throw bad_function_call();
 #else
     _VSTD::abort();

libcxx/test/libcxx/clang_tidy.sh.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // REQUIRES: has-clang-tidy
 // XFAIL: modules-build
 
-// RUN: clang-tidy %s --warnings-as-errors=* -header-filter=.* -- -Wno-unknown-warning-option %{compile_flags}
+// RUN: clang-tidy %s --warnings-as-errors=* -header-filter=.* -- -Wweak-vtables -Wno-unknown-warning-option %{compile_flags}
 // -Wno-unknown-warning-option tells clang-tidy to ignore '-W' command-line arguments that it doesn't know.
 // There are some GCC-specific ones where clang-tidy would warn otherwise.
 
 // Prevent <ext/hash_map> from generating deprecated warnings for this test.
 #if defined(__DEPRECATED)
 #    undef __DEPRECATED
 #endif