不能将显式默认函数声明为 constexpr，因为隐式声明不是 constexpr

Question

I'm trying to compile a C++ library (with gcc 5.3.1-14ubuntu2) and got this type of error:

> In file included from
> /root/pitchfork/workspace/unanimity/include/pacbio/consensus/ModelConfig.h:49:0,
>                  from /root/pitchfork/workspace/unanimity/src/models/P6C4NoCovModel.cpp:42:
> /root/pitchfork/workspace/unanimity/include/pacbio/data/internal/BaseEncoding.h:119:31:
> error: explicitly defaulted function 'constexpr
> PacBio::Data::detail::NCBI2na&
> PacBio::Data::detail::NCBI2na::operator=(const
> PacBio::Data::detail::NCBI2na&)' cannot be declared as constexpr
> because the implicit declaration is not constexpr:
>      inline constexpr NCBI2na& operator=(const NCBI2na&) = default;

The part of the code that cause trouble is :

class NCBI2na
{
public:
    static inline constexpr NCBI2na FromASCII(const char base) { return NCBI2na{base}; }
    static inline constexpr NCBI2na FromRaw(const uint8_t raw) { return NCBI2na{raw}; }

public:
    ~NCBI2na() = default;

    inline constexpr NCBI2na(const NCBI2na&) = default;
    inline constexpr NCBI2na(NCBI2na&&) = default;

    inline constexpr NCBI2na& operator=(const NCBI2na&) = default;
    inline constexpr NCBI2na& operator=(NCBI2na&&) = default;    
};

The part of the code that seems to causes trouble is the "= default". This may alos be related

I looked around but could not find a solution to this problem so far. Here are some similar questions that could help:

constexpr defining static data member of literal type that is declared const constructor of derived class cannot be constexpr if base class contains array member

Answer 1

This seems like a GCC bug. Assuming you compile as C++14, then the rules as written are these:

[dcl.constexpr]/3

The definition of a constexpr function shall satisfy the following constraints:

• it shall not be virtual
• its return type shall be a literal type;
• each of its parameter types shall be a literal type;
• its function-body shall be = delete, = default, or ...

All of the above are in fact satisfied in the code you've shown us. So your assignment operator definition is okay, and should be accepted as constexpr .

---

This code (once the error inducing static functions are commented out), is accepted by GCC 5.4.0 . So you can definitely chalk it up to a compiler bug.