`constexpr` code expected to compile in c++20 but it doesn't. Why?

Question

From here : https://github.com/PacktPublishing/C-Plus-Plus-20-Mastery---Pure-and-Unfiltered/blob/main/CoreLanguage/constexprChanges/constexprChanges/main.cpp

I got the following code snippet (I only changed &a with &b within the invocation of Invoke):

struct A {
    virtual int Get()const {
        return 1 ;
    }
};
struct B : A {

    constexpr int Get() const override {
        return 2 ;
    }
};
struct C : B {

    int Get() const override {
        return 3 ;
    }
};

constexpr int Invoke(A *a) {
    return a->Get() ;
}
int main() {
    A a ;
    B b ;
    C c ;
    static_assert(Invoke(&b) == 2) ;
    
}

It is my understanding that this code should compile just fine in C++20 since all the expressions involved in the static assert are constexpr. However when trying to compile using gcc 13.3.0 I get the following error:

<source>: In function 'int main()':
<source>:34:34: error: non-constant condition for static assertion
   34 |         static_assert(Invoke(&b) == 2) ;
      |                       ~~~~~~~~~~~^~~~
<source>:34:22:   in 'constexpr' expansion of 'Invoke((& b.B::<anonymous>))'
<source>:34:34: error: the value of 'b' is not usable in a constant expression
<source>:32:11: note: 'b' was not declared 'constexpr'
   32 |         B b ;
      |           ^
Compiler returned: 1

Now I did notice however the code in the github repo is for MVSC (since I see Visual Studio solution).

Can someone explain to me what's going on?

(A bit more context : I am looking in depth into C++20 and I am trying more or less to understand which resources are reliable).

Update:

The code actually compiles on x64 msvc v19.40 VS17.10

Answer 1

You can't call Get on an A*, it needs to be const.

constexpr int Invoke(const A *a) {
    return a->Get() ;
}

b also needs to be constexpr.

int main() {
    A a ;
    constexpr B b ;
    C c ;
    static_assert(Invoke(&b) == 2) ;
    
}

Answer 2

constexpr code expected to compile in c++20 but it doesn't. Why?

Because the code you're looking at has multiple errors:
PacktPublishing/C-Plus-Plus-20-Mastery---Pure-and-Unfiltered/issues/1

From the above bugreport:

1. It uses a which is not constexpr in a context where it must be.
2. This also means that Invoke must take a const A*.
3. The static_assert(Invoke(&a) == 2) ; should use b instead of a since B::Get is the function returning 2.

Also note that the author has confirmed the errors:

An old version of the file was uploaded by mistake. The correct file has been sent to the publisher and I'm waiting for them to upload it.