MSVC C++20 Bug: Flat Hash Map With Variant Compilation Failure

Oct 23, 2025 by SLV Team 63 views

Hey everyone! 👋 We've got a tricky situation on our hands. It seems there's a compilation issue when using absl::flat_hash_map with absl::variant in MSVC (Microsoft Visual C++) with C++20. Specifically, if the variant holds incomplete types, the code fails to compile. Let's dive into the details, the problem, how to reproduce it, and what we know about it so far. This is important stuff, so pay close attention, guys!

The Heart of the Problem: Compilation Errors

The core of the issue lies in how MSVC handles absl::flat_hash_map when it encounters a variant containing incomplete types under the /std:c++20 flag. As you know, C++20 brings a lot of new features and improvements, but sometimes it also exposes existing problems or introduces new ones. In this case, it's the latter. When the compiler tries to process the code, it throws a bunch of errors, as you can see in the provided compiler output (linked in the original report). The errors seem to be rooted in the IsLifetimeBoundAssignmentFrom functionality, which is part of the Abseil library's internal workings. The problem doesn't exist when you compile using C++17, so that tells us the issue is a regression of some sort. It seems C++20's stricter interpretation of certain language features is exposing a weakness in the current implementation of flat_hash_map when dealing with variants and incomplete types.

Now, let's get into the specifics of why this is happening. The compiler struggles when it encounters a variant that includes types that haven't been fully defined. In this particular scenario, the code defines a variant that can hold either struct A or struct B. Within struct B, there's a flat_hash_map where the values are of type Any (which is the absl::variant<A, B>). When the compiler tries to figure out how to manage these types, the incomplete definition of A seems to trigger the errors, which leads to compilation failure. That’s the crux of the problem, and understanding it is half the battle won. The devil is in the details, so let's carefully consider what's happening.

The implications of this bug are pretty serious. If your project uses flat_hash_map with variant and relies on C++20, you're going to hit this wall. This could disrupt builds, lead to time wasted on troubleshooting, and potentially delay project timelines. It's especially troublesome because the code should work. You'd expect that the Abseil library, which is designed to be robust and reliable, would handle this scenario without a hitch. Unfortunately, that isn’t currently the case, meaning that you and I, we have to find a workaround or wait for a fix.

Compilation Errors Explained

Let's break down the error messages a little bit. If you're familiar with compiler jargon, this might make more sense to you. These errors typically involve type deduction, assignment operators, and memory management. The compiler is trying to figure out how to copy, move, or otherwise manage the variant values inside the flat_hash_map. Because the types are incomplete, it gets confused. These errors manifest as cryptic messages about incomplete types, undefined behavior, or issues with the internal mechanics of the flat_hash_map implementation. In simple terms, the compiler is essentially saying, "Hey, I can't figure out how to deal with this, because I don't know enough about the types involved". If you dig into the compiler output (the link is in the original report), you'll see a long list of confusing error messages. They are all related to the same core problem of dealing with the variant and incomplete types. Debugging these errors can be challenging. You might see things like template instantiation failures or unexpected behavior in the standard library. Debugging this is a real headache. To get around this, you'll need a deeper understanding of the Abseil library, template metaprogramming, and the inner workings of the C++ compiler itself. That's why it's so important to have a clear understanding of the issue and what's causing it.

Reproducing the Bug: Step-by-Step

Reproducing the bug is thankfully straightforward. This will help you see the issue for yourself.

Get the Code: Grab the code snippet provided in the original report. It's a minimal, complete example that demonstrates the issue. The code includes the necessary headers from the Abseil library, defines two structs (A and B), and uses a variant (Any) to hold either A or B. It then creates a flat_hash_map inside struct B that uses the Any variant as its value type. Make sure you have all the necessary includes. This is essential for the code to compile. Otherwise, you'll be getting errors from the start. That could be very confusing. It is a very simple and small piece of code. This is very important when reporting a bug. It makes it easier to diagnose the problem. It allows others to reproduce it on their systems.
Set Up Your Environment: Make sure you have MSVC 2022 (version 17.14.18 or later) installed, along with CMake (version 3.26.6 or later). Also, ensure you have the Abseil library available in your project. This includes correctly configured paths so that the compiler can find the Abseil headers. If you use a build system like CMake, make sure you've correctly included the Abseil library and that your build environment is set up properly. If you are using another build system, make sure the include paths are properly set.
Configure CMake: Configure your CMakeLists.txt to use the C++20 standard (/std:c++20). This is a crucial step. Without this flag, the issue won't manifest itself. You might also want to set any other compiler flags that are relevant to your project. This might include optimization flags, debug flags, or other options. Setting the correct flags will ensure that the compiler behaves as expected.
Build the Code: Run the build process. This is the moment of truth. If everything is set up correctly, the build should fail with errors similar to those in the provided gist. If the build succeeds, you probably made a mistake in the previous steps. Double check all configurations. It is crucial to see those errors. Otherwise, you will be left in the dark wondering why your code isn't compiling.

If you get stuck, take a look at the provided compiler output (the gist). It'll give you a good idea of what to look for. If all goes well, you should see the same compilation errors as described in the report.

Deep Dive into the Code: Understanding the Structures

Let’s explore the code snippet. Understanding the code will help you understand the root of the issue. The code starts by including necessary headers for flat_hash_map and variant from the Abseil library. Then, it defines two structs, A and B, and creates a type alias called Any using absl::variant<A, B>. This means Any can hold either an A or a B instance. Finally, struct B contains a flat_hash_map where the keys are integers (int), and the values are of type Any. That is where our problem resides. The map is supposed to store either A or B instances, and this causes issues because struct A may not be fully defined when the flat_hash_map is being constructed or used.

The core problem arises from the interaction of incomplete types and the internal mechanisms of flat_hash_map. Because the compiler doesn't know the full details of struct A, it struggles to handle the variant<A, B> correctly. The flat_hash_map needs to know the size and layout of the value type to store and manage the data efficiently. When the type is incomplete, the compiler can't perform these tasks, leading to the compilation failures. It's like trying to build a house without knowing how many rooms it will have or what materials you will be using. You need the full picture before you can start construction. In our case, the compiler needs the full definition of struct A to build and use the flat_hash_map effectively.

Now, let's explore the implications of this problem and how it impacts your project. If you're working with C++20 and you are using absl::flat_hash_map with variant types that include incomplete types, then you're at risk of hitting this bug. This can affect projects of any size and scope. It may require careful planning and coordination to resolve this issue and keep your project moving forward. The goal is to either find a workaround or wait for a fix.

The Role of `absl::variant` and Incomplete Types

Let's get even deeper into this. The absl::variant type is a powerful tool for representing values that can hold different types. This is really useful. However, its interactions with incomplete types can be tricky. When you declare absl::variant<A, B>, you're saying the variant can hold either A or B. However, if A is not fully defined at the point where you create the variant, the compiler might struggle. In general, incomplete types are types whose definitions are not fully known at the point of use. This is a common situation. It might be due to forward declarations. This can often happen when you have circular dependencies between classes. While this is a common occurrence in C++, it often requires specific handling. The compiler needs to know details like the size and members of a type before it can use it effectively. If the compiler doesn't have this information, it will not know how to handle the variant correctly. This can manifest as compilation errors or unexpected behavior at runtime. Understanding this will help you understand the implications of the bug and how to avoid it.

Workarounds and Possible Solutions

Okay, so what can we do? We have a problem, but don't worry, there are a few options to consider:

Avoid Incomplete Types: The simplest (but not always feasible) workaround is to avoid using incomplete types within your variants. Make sure all the types are fully defined when you declare the variant. This means that if A and B are in different files, make sure the definition of A is included before the declaration of your variant. This can involve refactoring your code to ensure that all types are fully defined when the variant is used. This may be time-consuming. However, it can often solve the problem. If you can define all of your types at the point of use, you can side-step the issue. In some projects, this can be quite complex.
Use C++17: If possible, and if you don't need the specific features of C++20 that are causing you to use it, you can revert to C++17. This can buy you time to implement a more comprehensive solution. However, this depends on your project's needs and dependencies. This may not be an option for you. You will lose the benefits of C++20. This could be a deal-breaker if you need C++20 features. If you are stuck in this situation, you will need to think about the next workaround.
Manually Manage Storage: If you can't avoid incomplete types and can't go back to C++17, you could manually manage the storage for the variant values within the flat_hash_map. This would involve using raw pointers or custom allocators to control the memory and lifecycle of the objects. This is a very complex workaround. It involves a much higher level of manual memory management. This is definitely not for the faint of heart. Be careful if you take this route. You'll need to know a lot about memory management. This is also prone to errors and is time consuming. This will make your code more complex and harder to maintain.
Patch the Abseil Library: You can consider patching the Abseil library locally to address the issue. You could try commenting out IsLifetimeBoundAssignmentFrom as mentioned in the report, or you could try to modify the code in flat_hash_map to handle incomplete types more gracefully. Make sure you understand what you are doing. This is very risky and can have unintended consequences. You will need a thorough understanding of the Abseil library, template metaprogramming, and the C++ compiler. You should also create automated tests to ensure your changes didn't break anything else. This might involve diving into the Abseil source code and making changes. It is difficult to get right and can easily break things. Also, you may lose those changes when you update the library.
Wait for a Fix: The most straightforward solution is to wait for the Abseil developers to release a fix. Monitor the issue and keep an eye on the Abseil repository. When a fix is available, you can update your Abseil library and rebuild your project. This is the least risky solution, but it depends on the Abseil maintainers to address the issue.

Conclusion: Navigating the Compilation Maze

In conclusion, this compilation issue with absl::flat_hash_map and absl::variant in MSVC C++20 is a tricky one. We've explored the core problem, how to reproduce it, and various workarounds. Your best course of action depends on your project's specific needs, your time constraints, and your willingness to dive into the nitty-gritty details of the Abseil library. Carefully assess the trade-offs of each solution. Always test your changes thoroughly. If you are facing this problem, choose the solution that best fits your situation. Stay informed, and keep an eye on updates from the Abseil community. Good luck, and happy coding! 🚀