VS2019 Latest Update: constexpr: "expression did not evaluate to a constant" for FIXED string array

Question

^{Note: There are a number of questions on Stack Overflow with very similar-looking titles, but none that I have found is actually a duplicate, IMHO.}

I have been using code like the following in my project(s) for a number of years, without problem. However, since a recent update to Visual Studio 2019 (16.7.2 - though it may have been at 16.7.1), the MSVC compiler has started to generate the error shown (I have the compilation 'Standard' set to C++17).

#include <iostream>

class Foo {
public:
    Foo() { }
    static constexpr char Letters[6][10] = { "Alpha", "Bravo", "Charlie", "Delta", "Echo", "Foxtrot" };
};

int main()
{
    Foo f;
    for (int i = 0; i < 6; ++i) std::cout << f.Letters[i] << std::endl;
    return 0;
}

Error (at the opening brace in the constexpr line):

error C2131: expression did not evaluate to a constant
message : failure was caused by a read of an uninitialized symbol

The clang-cl compiler continues to accept the code without any warning.

I have a fairly 'trivial' fix for this issue, as below:

class Foo {
public:
    Foo() { }
    inline static const char Letters[6][10] = { "Alpha", "Bravo", "Charlie", "Delta", "Echo", "Foxtrot" };
};

However, I am intrigued by the error report. Is there something I am missing (and have been missing for ~5 years), or is this a bug in the latest release of MSVC? If the former, what is my error or invalid assumption?

Answer 1

Your code is valid (and accepted by GCC and Clang), and this is indeed a bug in MSVC v 19.27 and above. Online demo: https://gcc.godbolt.org/z/rdnn9M8j3

The bug was reported on January 26, 2021: https://developercommunity.visualstudio.com/t/Compile-error--when-declare-static-const/1320740 and is currently under investigation.