Why does clang, using libstdc++, delete the explicitly defaulted constructor on a type containing std::optional?

Consider the following structure with an std::optional containing a type that definitely has a "normal" default constructor.

#include <optional>
#include <string>

struct Foo
{
    Foo() = default;
    const std::optional<std::string> m_value;
};

bool function()
{
    Foo foo;
    return bool(foo.m_value);
}

Compiling the following with clang 9 (using the system's default libstdc++, for its gcc 8) gives an unexpected warning:

<source>:6:5: warning: explicitly defaulted default constructor is implicitly deleted [-Wdefaulted-function-deleted]
    Foo() = default;
    ^
<source>:7:38: note: default constructor of 'Foo' is implicitly deleted because field 'm_value' of const-qualified type 'const std::optional<std::string>' (aka 'const optional<basic_string<char> >') would not be initialized
    const std::optional<std::string> m_value;
                                     ^

There's also hard error for Foo foo; since it uses said deleted constructor.

Deleting the Foo() = default; constructor gives the same results.
Replacing it with Foo() {} works!
Removing all the constructors and initializing foo as Foo foo{}; works!
Explicitly initializing the member as const std::optional<std::string> m_value{}; works!
Removing the const from the member works! (but isn't the same meaning)
Using clang 9 with -stdlib=libc++ works!
Using gcc 8.3 (still with libstdc++) works!

I've read std::optional - construct empty with {} or std::nullopt? which seems to indicate that the libstdc++ implementation choice of an = default constructor for std::optional is likely to blame. But, in that question, the concern was a matter of efficiency of one approach vs. the other. In this case, it seems like a matter of correctness.

(I suspect that the answer to How can std::chrono::duration::duration() be constexpr? is going to be part of the story here.)

I see the same behaviour on Compiler Explorer: https://gcc.godbolt.org/z/Yj1o5P

Compare simple structures of optional and non-optional std::string (in the non-working configurations):

struct Foo { const std::optional<std::string> m_value; };
auto f1() { Foo f; return f.m_value; } // Fails: call to implicitly deleted constructor.

struct Bar { const std::string m_value; };
auto f2() { Bar b; return b.m_value; } // Works.

Is this a bug in libstdc++? Is it mixed intentions and assumptions between clang and libstdc++?

Surely the intent can't be that I can have a structure with a const std::string but I can't have a structure with a const std::optional<std::string> unless I wrote a constructor?

(In real-word cases, you'd have additional constructors, too. Thus the motivation for an = default() constructor in the first place. That, and clang-tidy.)

Edit: Here's an expanded version of the example (Compiler Explorer) showing a similar example working in "pure clang", "pure gcc", but failing in mixed "clang+libstdc++". This slightly bigger example is still artificial, but hints at why one might want to actually have such a defaulted constructor.

struct Foo
{
    Foo() = default; // Implicitly deleted?!
    explicit Foo(std::string arg) : m_value{std::move(arg)} {}
    const auto& get() const noexcept { return m_value; }
  private:    
    const std::optional<std::string> m_value;
};

// Maybe return an empty or a full Foo.
auto function(bool flag, std::string x)
{
    Foo foo1;
    Foo foo2{x};
    return flag ? foo1 : foo2;
}

This is a combination of:

an underspecification in the Standard;
a suboptimal Library implementation; and
a compiler bug.

Firstly, the Standard does not specify whether for the default optional.ctor a permissible implementation would be to defined it as defaulted:

constexpr optional() noexcept = default;
                              ^^^^^^^^^ OK?

Note that functions.within.classes answers the question in the affirmative for copy/move constructors, assignment operators, and non-virtual destructors, but does not mention default constructors.

This matters because it affects program correctness; on the assumption that optional has data members approximating the following:

template<class T>
class optional {
    alignas(T) byte buf[sizeof(T)]; // no NSDMI
    bool engaged = false;
    // ...
};

then since buf is a direct non-variant non-static data member lacking a default member initializer, if the default constructor of optional is defined as defaulted and thus is not user-provided, optional is not const-default-constructible and so optional<A> const a; is ill-formed.

It is thus a bad idea for a Library to define the default constructor of optional as defaulted, not only for this reason but also because it makes a value-initialized optional<B> b{}; perform more work than necessary, since it must zero-initialize buf, as observed std::optional - construct empty with {} or std::nullopt? - see in particular this answer. libstdc++ is fixed in this commit, which will be included in the next release of gcc, presumptively gcc 11.

Finally, it is a bug in gcc that it allows a non-const-default-constructible type for a const non-static data member without default member initializer of a class type whose default constructor is defined as defaulted; clang is correct to reject it. A reduced testcase is:

struct S {
    S() = default;
    int const i;
};

The best workaround in your case would be to supply a NSDMI:

const std::optional<std::string> m_value = std::nullopt;
                                         ^^^^^^^^^^^^^^

or (though I prefer the former, as it gives better codegen under Clang/libstdc++):

const std::optional<std::string> m_value = {};
                                         ^^^^

You might also consider giving Foo a user-defined default constructor; this results in better codegen under gcc (not zeroing the buffer but only setting the engaged member to false) thanks to what is presumably a related compiler bug.

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