g++-10 internal compiler error In member function ‘constexpr std::strong_ordering S9::operator<=>(const S9&) const’

[Damien Ruscoe]

Command Line:

g++-10 -std=c++20 -c strong_vs_weak_ordering.cpp

stderr:

strong_vs_weak_ordering.cpp: In member function ‘constexpr std::strong_ordering S9::operator<=>(const S9&) const’:
strong_vs_weak_ordering.cpp:69:26: internal compiler error: in finish_expr_stmt, at cp/semantics.c:681
   69 | std::strong_ordering operator<=>(const S9&) const = default;
      | ^~~~~~~~
0x7fdb22256082 __libc_start_main
 ../csu/libc-start.c:308
Please submit a full bug report,
with preprocessed source if appropriate.
Please include the complete backtrace with any bug report.
See <file:///usr/share/doc/gcc-10/README.Bugs> for instructions.

code:

#include<compare>

// not in our immediate control
struct Legacy
{
    bool operator==(Legacy const&) const;
    bool operator<(Legacy const&) const;
};

struct S6
{
    int x;
    Legacy l;
    // deleted because Legacy doesn't have operator<=>(), comparison category
    // can't be deduced
    auto operator<=>(const S6&) const = default;
};

struct S7
{
    int x;
    Legacy l;

    std::strong_ordering operator<=>(const S7& rhs) const = default;
    /*
    Since comparison category is provided explicitly, ordering can be
    synthesized using operator<() and operator==(). They must return exactly
    `bool` for this to work. It will work for weak and partial ordering as well.

    Here's an example of synthesized operator<=>():
    std::strong_ordering operator<=>(const S7& rhs) const
    {
        // use operator<=>() for int
        if(auto cmp = x <=> rhs.x; cmp != 0) return cmp;

        // synthesize ordering for Legacy using operator<() and operator==()
        if(l == rhs.l) return std::strong_ordering::equal;
        if(l < rhs.l) return std::strong_ordering::less;
        return std::strong_ordering::greater;
    }
    */
};

struct NoEqual
{
    bool operator<(const NoEqual&) const = default;
};

struct S8
{
    NoEqual n;
    // deleted, NoEqual doesn't have operator<=>()
    // auto operator<=>(const S8&) const = default;

    // deleted as well because NoEqual doesn't have operator==()
    std::strong_ordering operator<=>(const S8&) const = default;
};

struct W
{
    std::weak_ordering operator<=>(const W&) const = default;
};

struct S9
{
    W w;
    // ask for strong_ordering but W can provide only weak_ordering, this will
    // yield an error during instantiation
    std::strong_ordering operator<=>(const S9&) const = default;
    void f()
    {
        (S9{} <=> S9{}); // error
    }
};

[Matthias Klose (doko)]

this is fixed in GCC 11 and up:

tst.cc: In member function ‘void S9::f()’:
tst.cc:72:22: error: use of deleted function ‘constexpr std::strong_ordering S9::operator<=>(const S9&) const’
   72 | (S9{} <=> S9{}); // error
      | ^
tst.cc:69:26: note: ‘constexpr std::strong_ordering S9::operator<=>(const S9&) const’ is implicitly deleted because the default definition would be ill-formed:
   69 | std::strong_ordering operator<=>(const S9&) const = default;
      | ^~~~~~~~
tst.cc:66:7: error: three-way comparison of ‘S9::w’ has type ‘std::weak_ordering’, which does not convert to ‘std::strong_ordering’
   66 | W w;
      | ^