Coverity static analysis warning seen on latest (v30.2) message_lite.h

[kmurtty]

What version of protobuf and what language are you using?
Version: 6.30.2
Language: C++

What operating system (Linux, Windows, ...) and version?
Linux, v5.15.72, aarch64

What runtime / compiler are you using (e.g., python version or gcc version)
gcc

What did you do?

Ran Coverity static analysis of our software.

What did you expect to see

A clean Coverity run without any errors.

What did you see instead?

Coverity Static Analysis tool is complaining about the latest file “google/protobuf/message_lite.h” file. In particular, the syntax checker is complaining about the following lines of code in “message_lite.h”. The error message is saying that line number 284 is using an incomplete type.

Question: Can we safely ignore this error message? Any potential that this segment of code could lead to invalid code generation?

Following 7 lines are from “google/protobuf/message_lite.h”:

281 struct EnumTraitsImpl {
282 struct Undefined;
283 template
284 static Undefined value;
285 };
286 template
287 using EnumTraits = decltype(EnumTraitsImpl::value);

The full COVERITY warning message from our tool is as follows:

Type: Parse recovery warning (RW.INCOMPLETE_VAR_TYPE)
Classification: Unclassified
Severity: Unspecified
Action: Undecided
Owner: apotjenkins
Defect only exists locally.
/localdata/build/tmp/work/armv8a-linux/app-service/999-r0/recipe-sysroot/usr/include/google/protobuf/message_lite.h:284:3:

1. incomplete_var_type: variable cannot have incomplete type "google::protobuf::internal::EnumTraitsImpl::Undefined"

Make sure you include information that can help us debug (full error message, exception listing, stack trace, logs).

Anything else we should know about your project / environment

We are using gRPC Library version 1.71.0.

[acozzette]

Yes, this is safe to ignore. I'm not surprised that Coverity would complain about it because it is an unusual construction, but it's harmless.

[Almskou]

This issue causes the nvcc compiler to return an error.

Simplified example:

file: example.cpp

struct EnumTraitsImpl {
    struct Undefined;
    template<typename T>
    static Undefined value;
};

template<typename T>
using EnumTraits = decltype(EnumTraitsImpl::value<T>);

int main() {
    EnumTraits<int> a;
}

Compiled with nvcc:

$ nvcc example.cu -arch=sm_86
example.cu(4): error: variable cannot have incomplete type "EnumTraitsImpl::Undefined"
      static Undefined value;
      ^

example.cu(11): error: incomplete type "EnumTraits<int>" (aka "EnumTraitsImpl::Undefined") is not allowed
      EnumTraits<int> a;
                      ^

Compiled with gcc:

$gcc example.cpp
example.cpp: In function 'int main()':
example.cpp:11:21: error: aggregate 'EnumTraits<int> a' has incomplete type and cannot be defined
   11 |     EnumTraits<int> a;
      |                     ^

Versions

$nvcc --version 
nvcc: NVIDIA (R) Cuda compiler driver
Copyright (c) 2005-2025 NVIDIA Corporation
Built on Fri_Feb_21_20:23:50_PST_2025
Cuda compilation tools, release 12.8, V12.8.93
Build cuda_12.8.r12.8/compiler.35583870_0


$gcc --version
gcc (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0
Copyright (C) 2023 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Expected
That both compiler to fail with the same error.

I ran into this problem when I was trying to bump Protobuf from version 28.3 to 30.2

[sbenzaquen]

That simplified example shows a problem elsewhere.
EnumTraits<int> is wrong because the trait is not defined for int.
It is only defined for proto enums.
If you see it as undefined, maybe you are missing the #include for the particular header that defines the enum?

[Almskou]

I know the usage of EnumTraits<int> is wrong, but I used it to provoke the error: variable cannot have incomplete type "EnumTraitsImpl::Undefined" when compiling with nvcc.

The implementation of EnumTraitsImpl is the same as here:
https://github.com/protocolbuffers/protobuf/blob/main/src/google/protobuf/message_lite.h#L284

The error message I get from my project where I want to bump protobuf from 28.3 to 30.2 or newer.

bazel-out/k8-fastbuild/bin/external/protobuf~/src/google/protobuf/_virtual_includes/protobuf_lite/google/protobuf/message_lite.h(284): error: variable cannot have incomplete type "google::protobuf::internal::EnumTraitsImpl::Undefined"
    static Undefined value;
    ^

This error does not come when I use protobuf 28.3 in my project.

Both compiled with nvcc