proxy-types: fix clang-tidy StackAddressEscape error

This error should not be a real problem because code is taking an invalid
reference to an empty object that has no state and could never be used. But
taking the reference could technically be undefined behavior.

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

include/mp/proxy-types.h:134:5: error: Address of stack memory associated with temporary object of type '(lambda at include/mp/proxy-types.h:134:51)' is still referred to by a temporary object on the stack upon returning to the caller.  This will be a dangling reference [clang-analyzer-core.StackAddressEscape,-warnings-as-errors]
  134 |     return ReadDestEmplace{TypeList<LocalType>(), [&](auto&&... args) -> decltype(auto) {
      |     ^
build/test/mp/test/foo.capnp.proxy-server.c++:51:12: note: Calling 'serverInvoke<mp::ProxyServer<mp::test::messages::FooInterface>, capnp::CallContext<mp::test::messages::FooInterface::PassCustomParams, mp::test::messages::FooInterface::PassCustomResults>, mp::ServerField<1, mp::Accessor<mp::foo_fields::Arg, 17>, mp::ServerRet<mp::Accessor<mp::foo_fields::Result, 18>, mp::ServerCall>>>'
   51 |     return serverInvoke(*this, call_context, MakeServerField<1, Accessor<foo_fields::Arg, FIELD_IN | FIELD_BOXED>>(Make<ServerRet, Accessor<foo_fields::Result, FIELD_OUT | FIELD_BOXED>>(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 true
  700 |     if constexpr (std::is_same_v<decltype(fn()), void>) {
      |                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/mp/proxy-types.h:700:5: note: Taking true branch
  700 |     if constexpr (std::is_same_v<decltype(fn()), void>) {
      |     ^
include/mp/proxy-types.h:701:9: note: Calling 'operator()'
  701 |         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::Arg, 17>, mp::test::FooCustom, mp::ServerInvokeContext<mp::ProxyServer<mp::test::messages::FooInterface>, capnp::CallContext<mp::test::messages::FooInterface::PassCustomParams, mp::test::messages::FooInterface::PassCustomResults>>, const mp::ServerRet<mp::Accessor<mp::foo_fields::Result, 18>, 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/proxy-types.h:304:5: note: Calling 'MaybeReadField<mp::TypeList<mp::test::FooCustom>, mp::InvokeContext &, mp::StructField<mp::Accessor<mp::foo_fields::Arg, 17>, const mp::test::messages::FooInterface::PassCustomParams::Reader>, mp::ReadDestEmplace<mp::test::FooCustom, (lambda at include/mp/proxy-types.h:305:83)>>'
  304 |     MaybeReadField(std::integral_constant<bool, Accessor::in>(), TypeList<ArgType>(), invoke_context,
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  305 |         Make<StructField, Accessor>(params), ReadDestEmplace(TypeList<ArgType>(), [&](auto&&... args) -> auto& {
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  306 |             param.emplace(std::forward<decltype(args)>(args)...);
      |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  307 |             return *param;
      |             ~~~~~~~~~~~~~~
  308 |         }));
      |         ~~~
include/mp/proxy-types.h:276:5: note: Calling 'ReadField<mp::test::FooCustom, mp::InvokeContext &, mp::StructField<mp::Accessor<mp::foo_fields::Arg, 17>, const mp::test::messages::FooInterface::PassCustomParams::Reader>, mp::ReadDestEmplace<mp::test::FooCustom, (lambda at include/mp/proxy-types.h:305:83)>>'
  276 |     ReadField(std::forward<Args>(args)...);
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/mp/proxy-types.h:175:12: note: Calling 'CustomReadField<mp::StructField<mp::Accessor<mp::foo_fields::Arg, 17>, const mp::test::messages::FooInterface::PassCustomParams::Reader>, mp::ReadDestEmplace<mp::test::FooCustom, (lambda at include/mp/proxy-types.h:305:83)>>'
  175 |     return CustomReadField(TypeList<RemoveCvRef<LocalTypes>...>(), Priority<2>(), std::forward<Args>(args)...);
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test/mp/test/foo-types.h:51:12: note: Calling 'ReadDestEmplace::update'
   51 |     return read_dest.update([&](FooCustom& value) {
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   52 |         value.v1 = ReadField(TypeList<std::string>(), invoke_context, mp::Make<mp::ValueField>(custom.getV1()), ReadDestTemp<std::string>());
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   53 |         value.v2 = custom.getV2();
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~
   54 |     });
      |     ~~
include/mp/proxy-types.h:112:23: note: 'is_const_v' is false
  112 |         if constexpr (std::is_const_v<std::remove_reference_t<std::invoke_result_t<EmplaceFn>>>) {
      |                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/mp/proxy-types.h:112:9: note: Taking false branch
  112 |         if constexpr (std::is_const_v<std::remove_reference_t<std::invoke_result_t<EmplaceFn>>>) {
      |         ^
include/mp/proxy-types.h:122:13: note: Calling 'operator()'
  122 |             update_fn(temp);
      |             ^~~~~~~~~~~~~~~
test/mp/test/foo-types.h:52:113: note: Calling 'ReadDestTemp<std::basic_string<char>>'
   52 |         value.v1 = ReadField(TypeList<std::string>(), invoke_context, mp::Make<mp::ValueField>(custom.getV1()), ReadDestTemp<std::string>());
      |                                                                                                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~
include/mp/proxy-types.h:134:5: note: Address of stack memory associated with temporary object of type '(lambda at include/mp/proxy-types.h:134:51)' is still referred to by a temporary object on the stack upon returning to the caller.  This will be a dangling reference
  134 |     return ReadDestEmplace{TypeList<LocalType>(), [&](auto&&... args) -> decltype(auto) {
      |     ^                                             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  135 |         return LocalType{std::forward<decltype(args)>(args)...};
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  136 |     }};
      |     ~

Author: ryanofsky

.clang-tidy

@@ -26,6 +26,7 @@ performance-*,
 readability-const-return-type,
 readability-redundant-declaration,
 readability-redundant-string-init,
+clang-analyzer-core.StackAddressEscape,
 '
 HeaderFilterRegex: '.'
 WarningsAsErrors: '*'

include/mp/proxy-types.h

@@ -92,7 +92,7 @@ struct StructField
 template <typename LocalType, typename EmplaceFn>
 struct ReadDestEmplace
 {
-    ReadDestEmplace(TypeList<LocalType>, EmplaceFn&& emplace_fn) : m_emplace_fn(emplace_fn) {}
+    ReadDestEmplace(TypeList<LocalType>, EmplaceFn emplace_fn) : m_emplace_fn(std::move(emplace_fn)) {}
 
     //! Simple case. If ReadField impementation calls this construct() method
     //! with constructor arguments, just pass them on to the emplace function.
@@ -123,15 +123,15 @@ struct ReadDestEmplace
             return temp;
         }
     }
-    EmplaceFn& m_emplace_fn;
+    EmplaceFn m_emplace_fn;
 };
 
 //! Helper function to create a ReadDestEmplace object that constructs a
 //! temporary, ReadField can return.
 template <typename LocalType>
 auto ReadDestTemp()
 {
-    return ReadDestEmplace{TypeList<LocalType>(), [&](auto&&... args) -> decltype(auto) {
+    return ReadDestEmplace{TypeList<LocalType>(), [](auto&&... args) -> decltype(auto) {
         return LocalType{std::forward<decltype(args)>(args)...};
     }};
 }