MSVC /permissive- leads to "expression did not evaluate to a constant"

[sdebionne]

In some circumstance -I have a hard time reducing the repro, compiling with MSVC 2019 and /std=c++17 /permissive-, I get error C2131.

Here is the repro I have so far:

#include <string>

/// Returns the length of a C style array (that has not decayed into a pointer)
template <typename T, std::size_t sz>
inline constexpr std::size_t lengthof(T (&)[sz])
{
    return sz;
}

#include "https://raw.githubusercontent.com/aantron/better-enums/0.11.3/enum.h"

namespace foo {

BETTER_ENUM(bar, int,
           one,
           two
);

} //namespace foo

and the result:

<source>(15): error C2131: expression did not evaluate to a constant
<source>(15): note: failure was caused by call of undefined function or one not declared 'constexpr'
<source>(15): note: see usage of 'foo::bar::bar'

Easily reproducible on godbolt.

This really looks like a compiler bug (the enum constructor not being seen as constexpr), but given the difficulty to reduce the repro, I cannot tell what it is and if there is workaround...

[sdebionne]

I have finally managed to reduce the issue -without better enum.

#include <cstddef>

/// Returns the length of a C style array (that has not decayed into a pointer)
template <typename T, std::size_t sz>
inline constexpr std::size_t lengthof(T (&)[sz])
{
    return sz;
}

struct foo
{
    struct bar {};
    constexpr explicit foo(bar) {}
};
inline constexpr foo foo{foo::bar{}};

[sdebionne]

Reported to MS Community

[aantron]

Thanks!