Accessing constant variables from a lambda in Visual Studio 2026

Question

I have a program that I would like to build in Visual Studio 2026, but struggle with the errors appearing.

One simplified piece of code is as follows:

void f(const void*) {}

int main() {
    constexpr float one[1] = { 1.0f };
    [&]() {
        f(one);
        f(&one[0]);
    }();
}

In /std:c++17 mode, MSVC accepts the program just fine.
In /std:c++20 mode, MSVC shows two errors:

<source>(6): error C2326: 'auto main::<lambda_1>::operator ()(void) const': function cannot access 'one'
<source>(7): error C2101: '&' on constant

and in /std:c++latest mode, it shows only error C2101.

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What changed in C++20 and later versions that caused the program to be rejected?

In MSVC, /std:c++17 alone does not compile the code by the C++17 standard, it has an implicit /permissive which allows for Microsoft extensions. If you add /permissive- to the compile flags, the code won't compile. MSVC /std:c++20 changed /permissive- (Microsoft extensions (mostly) disable) to be the default. — Eljay
– Eljay, Commented Feb 22 at 18:35

Jarod42 · Accepted Answer · 2026-02-23 09:46:03Z

4

What changed in C++20 and later versions that caused the program to be rejected?

Flag /permissive- is enabled by default from C++20 in msvc. Microsoft has many extensions that results in successful compilation of an invalid C++ program. If you use /permissive- flag with C++17, you'll notice that msvc also starts rejecting the said program(which is incorrect behavior as the program is well-formed). This means that msvc was already rejecting the program(as far as standard compliance is concerned)

There is a closely related msvc bug that was fixed just 4 days ago(on 19-feb-2026). The reported bug says:

A fix for this issue is now available in preview release. Try out the fix by installing the most recent preview from  https://visualstudio.microsoft.com/vs/preview/.

edited Feb 23 at 9:46

Jarod42

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answered Feb 23 at 2:28

Anoop Rana

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Comments

0___________ · Accepted Answer · 2026-02-22 19:14:57Z

0

In C++20, even if a variable is constexpr, if you take its address or pass an array (which decays to a pointer), it is "ODR-used." This requires the variable to be captured.

void f(const void*) {}

int main() {
    constexpr float one[1] = { 1.0f };
    
    // Capture the pointer to the array explicitly
    [ptr = one]() {
        f(ptr);
        f(&ptr[0]);
    }();
}

edited Feb 22 at 19:14

answered Feb 22 at 18:41

0___________

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user7860670 Feb 22 at 19:35

But OP already does capture the variable by refence. And other compilers compile his code just fine godbolt.org/z/5G98nWP9f

0___________ Feb 22 at 20:15

I think we was very specific about the compiler. What your exmaple fhave in common with msvc

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