qvariant.h:710: warning: C4702: unreachable code

Robert Hairgrove

@Christian-Ehrlicher said in qvariant.h:710: warning: C4702: unreachable code:

What exact Qt version do you use and where in the header does it occour?

This occurs with the following Qt versions:
6.5.3
6.7.3
6.8.3
6.9.2
6.9.3
6.10.0
6.10.1
6.10.2
6.11.0

As I wrote in the subject to this thread, it occurs at line 710 in qvariant.h.

Christian Ehrlicher

What MSVC version do you use? I don't get a warning there with /W4

    template<typename T>
#ifndef Q_QDOC
    static inline auto fromValue(const T &value)
        noexcept(std::is_nothrow_copy_constructible_v<T> && Private::CanUseInternalSpace<T>)
        -> std::enable_if_t<std::is_copy_constructible_v<T> && std::is_destructible_v<T>, QVariant>
#else
    static inline QVariant fromValue(const T &value)
#endif
    {
        if constexpr (std::is_null_pointer_v<T>)
            return QVariant(QMetaType::fromType<std::nullptr_t>());
        else if constexpr (std::is_same_v<T, QVariant>) <<-- this is line 740
            return value;
        else if constexpr (std::is_same_v<T, std::monostate>)
            return QVariant();
        return QVariant(QMetaType::fromType<T>(), std::addressof(value));
    }

I don't see why the compiler should throw an error here.

Robert Hairgrove

This is the relevant part of qvariant.h starting at line 695:

    template<typename T>
#ifndef Q_QDOC
    static inline auto fromValue(const T &value)
        noexcept(std::is_nothrow_copy_constructible_v<T> && Private::CanUseInternalSpace<T>)
        -> std::enable_if_t<std::is_copy_constructible_v<T> && std::is_destructible_v<T>, QVariant>
#else
    static inline QVariant fromValue(const T &value)
#endif
    {
        if constexpr (std::is_null_pointer_v<T>)
            return QVariant(QMetaType::fromType<std::nullptr_t>());
        else if constexpr (std::is_same_v<T, QVariant>)
            return value;
        else if constexpr (std::is_same_v<T, std::monostate>)
            return QVariant();
// <== WARNING: Unreachable code occurs on the next line:
        return QVariant(QMetaType::fromType<T>(), std::addressof(value));
    }

Christian Ehrlicher

You should take a look from where excatly it comes as this part is for sure not unreachable. What exact MSVC version do you use?

Robert Hairgrove

@Christian-Ehrlicher said in qvariant.h:710: warning: C4702: unreachable code:

What MSVC version do you use?

This happens using two different compiler versions:

MSVC 2022 Community v17.14.25 - cl.exe version 19.44.35222 for x64

and

MSVC 2026 Community v18.2.1 - cl.exe version 19.50.35723 for x64

Both are done in different instances of Oracle VirtualBox running on Linux Ubuntu 24.04 hosts with Windows 10 guests.

It doesn't surprise me that you cannot duplicate the warning, because it appears that QVariant must be instantiated in a certain way in order for the warning to occur. And that is my problem here because the diagnostic MSVC gives me does not show where in my own code that this is being triggered, AFAICT.

Christian Ehrlicher

So you should show use your testcase where it fails ...

Christian Ehrlicher

You maybe can wrap the return with an else

        else if constexpr (std::is_same_v<T, std::monostate>)
            return QVariant();
        else
            return QVariant(QMetaType::fromType<T>(), std::addressof(value));

Robert Hairgrove

@Christian-Ehrlicher said in qvariant.h:710: warning: C4702: unreachable code:

You maybe can wrap the return with an else

        else if constexpr (std::is_same_v<T, std::monostate>)
            return QVariant();
        else
            return QVariant(QMetaType::fromType<T>(), std::addressof(value));

I do not want to change the Qt source code! 😲

BTW, since in my project the symbol Q_QDOC is not defined, the elsebranch is grayed out ...

Robert Hairgrove

@Christian-Ehrlicher I will repeat what I just wrote:

And that is my problem here because the diagnostic MSVC gives me does not show where in my own code that this is being triggered

If I knew where, I could show you.

Christian Ehrlicher

@Robert-Hairgrove said in qvariant.h:710: warning: C4702: unreachable code:

If I knew where, I could show you.

That's the life of a developer...

Robert Hairgrove

I neglected to say that I am building with std:c++17, if that makes any difference.

Christian Ehrlicher

Did you try my proposed change?

Robert Hairgrove

@Christian-Ehrlicher said in qvariant.h:710: warning: C4702: unreachable code:

Did you try my proposed change?

No, because I do not want to start hacking the Qt source code.

Robert Hairgrove

In the meantime, I was able to narrow things down by doing make clean and compiling each source file individually. I found one file which makes heavy use of QVariant, and I was able to disable the warning in that file only. Now I have to run a full build to see if there are other places where the warning is triggered. But since I only get one warning, I think this might be the only place.

If that is the case, I will come back and mark the thread as "SOLVED".

JonB

@Robert-Hairgrove
Isn't your error Wrong C4702 unreachable code in templated early if constexpr return

This is reduced from the Qt 6.8.3 system header QVariant.

?

And assuming it is, as i understand it it is a MSVC not a Qt issue. For which there appears to be a fix release/patch:

A fix for this issue has been released! Install the most recent release from https://visualstudio.microsoft.com/downloads/. Thank you for providing valuable feedback which has helped improve the product.

That was in answer to someone using

cl /std:c++20 Project1.cpp

Microsoft (R) C/C++ Optimizing Compiler Version 19.44.35207.1 for x64

Robert Hairgrove

This post is deleted!

Robert Hairgrove

@JonB Thanks for the feedback. It is exactly like the first link, which was reported in October 2025 after the fix for the other issue appeared in March 2025.

The two issues are not related. The later issue, which is what I have, is still under investigation.

Christian Ehrlicher

@Robert-Hairgrove said in qvariant.h:710: warning: C4702: unreachable code:

No, because I do not want to start hacking the Qt source code.

This is stupid - you should not hack the source code but just try it out so we can change the code if it works.
So you have to life with the hack - but stop complaining about Qt not fixing your stuff.

JonB

@Robert-Hairgrove said in qvariant.h:710: warning: C4702: unreachable code:

The later issue, which is what I have, is still under investigation.

OIC :(

Christian Ehrlicher

@JonB said in qvariant.h:710: warning: C4702: unreachable code:

Isn't your error Wrong C4702 unreachable code in templated early if constexpr return
This is reduced from the Qt 6.8.3 system header QVariant.
?

And assuming it is, as i understand it it is a MSVC not a Qt issue. For which there appears to be a fix release/patch:

Yes, it is fixed in the current MSVC version. Thx for the investigation.