type-context: disable clang-tidy UndefinedBinaryOperatorResult error

Reported by fanquake <[email protected]>
bitcoin/bitcoin#31802 (comment)
https://cirrus-ci.com/task/6552721135763456
https://api.cirrus-ci.com/v1/task/6552721135763456/logs/ci.log

Error is spurious and comes from kj/async-inl.h and it seems to be shown
because clang-tidy considers this error to come from "main file" of the
translation unit (see https://clang.llvm.org/extra/clang-tidy/,
https://stackoverflow.com/a/47611238, https://reviews.llvm.org/D26418) even
though this is not the case.

Even though the header is included via -isystem and clang-tidy
--dump-config shows "SystemHeaders: false" the error is still shown.

I also tried to suppress the error by editing .clang-tidy to include:

  HeaderFilterRegex: '.*'
  ExcludeHeaderFilterRegex: '.*/include/kj/async-inl\.h$'

and many variations but the error is always shown, I assume because
ExcludeHeaderFilterRegex does not ignore the isInMainFile condition
(https://github.com/llvm/llvm-project/pull/91400/files).

The actual warning is a false positive that comes from ABI-specific code in
Cap'n Proto that gets the starting function address (that can be passed to
addr2line) from a lambda or function object. This code calls a helper to get
the starting function address from a pointer-to-member-function, which in this
case is the the operator() member function. That code handles pointers to
virtual member functions, so it checks if the pointer is virtual by testing its
low-order bit, and if set, assumes the first bytes of the object are a vtable
pointer, and does pointer arithmetic with the vtable address. Clang-tidy
complains about this because it does not know the vtable address is valid,
assuming incorrectly it is a "garbage value".

A possible way to address this might be to submit a patch to Cap'n Proto that
skips this code when __clang_analyzer__ is set. Maybe it could also be reported
as an analyzer bug https://clang-analyzer.llvm.org/
https://clang.llvm.org/docs/analyzer/user-docs/FAQ.html#excluding-code-from-analysis

Error output is:

/nix/store/46kiq9naswgbqfc14kc9nxcbgd0rv0m2-capnproto-1.1.0/include/kj/async-inl.h:609:37: error: The left operand of '+' is a garbage value [clang-analyzer-core.UndefinedBinaryOperatorResult,-warnings-as-errors]
  609 |       return *(void**)(*(char**)obj + voff);
      |                                     ^
build/test/mp/test/foo.capnp.proxy-server.c++:93:12: note: Calling 'serverInvoke<mp::ProxyServer<mp::test::messages::FooFn>, capnp::CallContext<mp::test::messages::FooFn::CallParams, mp::test::messages::FooFn::CallResults>, mp::ServerField<0, mp::Accessor<mp::foo_fields::Context, 17>, mp::ServerRet<mp::Accessor<mp::foo_fields::Result, 2>, mp::ServerCall>>>'
   93 |     return serverInvoke(*this, call_context, MakeServerField<0, Accessor<foo_fields::Context, FIELD_IN | FIELD_BOXED>>(Make<ServerRet, Accessor<foo_fields::Result, FIELD_OUT>>(ServerCall())));
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/mp/proxy-types.h:739:16: note: Calling 'ReplaceVoid<(lambda at include/mp/proxy-types.h:739:28), (lambda at include/mp/proxy-types.h:740:13)>'
  739 |         return ReplaceVoid([&]() { return fn.invoke(server_context, ArgList()); },
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  740 |             [&]() { return kj::Promise<CallContext>(kj::mv(call_context)); })
      |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/mp/proxy-types.h:700:19: note: 'is_same_v' is false
  700 |     if constexpr (std::is_same_v<decltype(fn()), void>) {
      |                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/mp/proxy-types.h:700:5: note: Taking false branch
  700 |     if constexpr (std::is_same_v<decltype(fn()), void>) {
      |     ^
include/mp/proxy-types.h:704:16: note: Calling 'operator()'
  704 |         return fn();
      |                ^~~~
include/mp/proxy-types.h:739:43: note: Calling 'ServerField::invoke'
  739 |         return ReplaceVoid([&]() { return fn.invoke(server_context, ArgList()); },
      |                                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/mp/proxy-types.h:563:16: note: Calling 'PassField<mp::Accessor<mp::foo_fields::Context, 17>, mp::ServerInvokeContext<mp::ProxyServer<mp::test::messages::FooFn>, capnp::CallContext<mp::test::messages::FooFn::CallParams, mp::test::messages::FooFn::CallResults>>, mp::ServerRet<mp::Accessor<mp::foo_fields::Result, 2>, mp::ServerCall>, mp::TypeList<>>'
  563 |         return PassField<Accessor>(Priority<2>(),
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  564 |             typename Split<argc, ArgTypes>::First(),
      |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  565 |             server_context,
      |             ~~~~~~~~~~~~~~~
  566 |             this->parent(),
      |             ~~~~~~~~~~~~~~~
  567 |             typename Split<argc, ArgTypes>::Second(),
      |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  568 |             std::forward<Args>(args)...);
      |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/mp/type-context.h:151:12: note: Calling 'CapabilityServerSet::getLocalServer'
  151 |     return server.m_context.connection->m_threads.getLocalServer(thread_client)
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/nix/store/46kiq9naswgbqfc14kc9nxcbgd0rv0m2-capnproto-1.1.0/include/capnp/capability.h:1274:10: note: Calling 'Promise::then'
 1274 |   return getLocalServerInternal(client)
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1275 |       .then([](void* server) -> kj::Maybe<typename T::Server&> {
      |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1276 |     if (server == nullptr) {
      |     ~~~~~~~~~~~~~~~~~~~~~~~~
 1277 |       return nullptr;
      |       ~~~~~~~~~~~~~~~
 1278 |     } else {
      |     ~~~~~~~~
 1279 |       return *reinterpret_cast<typename T::Server*>(server);
      |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1280 |     }
      |     ~
 1281 |   });
      |   ~~
/nix/store/46kiq9naswgbqfc14kc9nxcbgd0rv0m2-capnproto-1.1.0/include/kj/async-inl.h:1295:32: note: Calling 'GetFunctorStartAddress::apply'
 1295 |   void* continuationTracePtr = _::GetFunctorStartAddress<_::FixVoid<T>&&>::apply(func);
      |                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/nix/store/46kiq9naswgbqfc14kc9nxcbgd0rv0m2-capnproto-1.1.0/include/kj/async-inl.h:677:12: note: Calling 'PtmfHelper::apply'
  677 |     return PtmfHelper::from<ReturnType, Decay<Func>, ParamTypes...>(
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  678 |         &Decay<Func>::operator()).apply(&func);
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/nix/store/46kiq9naswgbqfc14kc9nxcbgd0rv0m2-capnproto-1.1.0/include/kj/async-inl.h:606:9: note: Assuming the condition is true
  606 |     if (voff & 1) {
      |         ^~~~~~~~
/nix/store/46kiq9naswgbqfc14kc9nxcbgd0rv0m2-capnproto-1.1.0/include/kj/async-inl.h:606:5: note: Taking true branch
  606 |     if (voff & 1) {
      |     ^
/nix/store/46kiq9naswgbqfc14kc9nxcbgd0rv0m2-capnproto-1.1.0/include/kj/async-inl.h:609:37: note: The left operand of '+' is a garbage value
  609 |       return *(void**)(*(char**)obj + voff);
      |                        ~~~~~~~~~~~~ ^

Author: ryanofsky

.clang-tidy

@@ -26,8 +26,9 @@ performance-*,
 readability-const-return-type,
 readability-redundant-declaration,
 readability-redundant-string-init,
-clang-analyzer-core.StackAddressEscape,
-clang-analyzer-optin.core.EnumCastOutOfRange,
+clang-analyzer-core.*,
+-clang-analyzer-core.UndefinedBinaryOperatorResult,
+clang-analyzer-optin.core.*,
 '
 HeaderFilterRegex: '.'
 WarningsAsErrors: '*'