Incomplete type works with gcc but not with clang and msvc答案

【问题标题】：Incomplete type works with gcc but not with clang and msvcIncomplete type works with gcc but not with clang and msvc
【发布时间】：2022-12-01 23:41:56
【问题描述】：

I have recently learnt about incomplete types and that under certain conditions they can be used as template arguments. In particular, like void, struct incomplete; are both incomplete types. Then I wrote the following program that works with gcc but not with msvc and clang. Live demo

struct incomplete;
template<typename T> struct C
{
    static constexpr T t{};
};

template<class T>
struct myClass {
    C<T> new_t() { return {}; }
};

int main() {
    myClass<incomplete> d;
    d.new_t();    
}

As we can see the above program compiles with gcc but not with msvc and clang. So I want to know which is the correct technical behavior.

Clang says:

<source>:4:24: error: constexpr variable cannot have non-literal type 'const incomplete'
    static constexpr T t{};

while msvc says:

<source>(4): error C2027: use of undefined type 'incomplete'
<source>(1): note: see declaration of 'incomplete'

while GCC accepts the code with both c++17 as well as c++20.

Which compiler is correct here?

【问题讨论】：

Interestingly, as soon as you odr-use t, gcc also rejects the code. This might be an indication as to why gcc accepts it in the first place.
Looks like ill-formed.
You need to "complete" the type eventually. Else the program is ill-formed.

标签： c++ language-lawyer c++20

【解决方案1】：

The program isill-formedandgcc is wrong in accepting the codebecause even though thedefinitionof the static data member is not instantiated(because it is not odr-used), it'sdeclarationwill still be instantiated due to theimplicit instantiationof C<incomplete> as per temp.inst#3. More importantly, if adeclarationuses constexpr or inline (since C++17) specifier, the member must be declared to have complete type.

From temp.inst#3:

The implicit instantiation of a class template specialization causes:

theimplicit instantiation of the declarations, but not of the definitions, of thenon-deleted class member functions, member classes, scoped member enumerations,static data members, member templates, and friends; and

(emphasis mine)

This means that the implicit instantiation C<incomplete> will cause the implicit instantiation of thedeclarationof the static data member.

Further from static data member documentation:

However, if the declaration uses constexpr or inline (since C++17) specifier, the membermust be declared to have complete type.

(emphasis mine)

This means that since the implicitly instantiated declaration, declares a member of an incomplete type, the program isill-formed.

Here is the gcc bug report:

GCC accepts invalid program involving constexpr static data member of class template

【讨论】：

Yes, I think [temp.inst] is key to deciding whether [basic.def] even applies. +1
@bitmask Thanks, here is the gcc bug report.

【解决方案2】：

According to [basic.def] 5. this is ill formed:

In the definition of an object, the type of that object shall not be an incomplete type ([basic.types.general]), an abstract class type ([class.abstract]), or a (possibly multi-dimensional) array thereof.

This does not seem to make any allowances for non-odr-used members of class templates.

【讨论】：