decltype がサイズが 0 の配列への参照を作れる / decltype can generate references of zero sized array by はぴぴ

Thank you for reacting.

Hello,

Thank you for reporting these issues. C2265 is no longer given on that line, we've added functionality to compiler to allow references to zero-length array. However, that last error message on calling f2(a) is not a legal C++ code. Compiler cannot deduce T in the function parameter type from the argument list. Please explicitly define the argument correctly on calling f2().

Thanks,
Ulzii Luvsanbat
Visual C++ Team

この度はフィードバックをお送りいただき、誠にありがとうございます。 現在、お送りいただきました報告内容に基づき問題を確認中ですので、しばらくお待ちください。 よろしくお願いいたします。

[Problem Description]:
I use Visual C++ 2010 Express. typedef char A[]; typedef A &RA; The above returns an error message C2265: reference of array which size is zero is invalid. extern char a[]; typedef decltype((a)) RA2; The above can create type char (&)[] without generating an error. This suggests that it is possible to internally create the type char (&)[]. https://connect.microsoft.com/VisualStudio/feedback/details/724314/t-const-char-t-const-cannot-receive-char In the above URL, I mentioned the problem: char[] cannot be converted to T const &. I use decltype as one of detouring tool. However, because this tool requires declaration of variable of type char [], T const (&)[] for a custom type T cannot be created. Therefore I believe that declaration of type T (&)[] without decltype is still necessary.

[Repro Steps]:
extern char a[];
typedef char A[];
//reference to zero-sized array is invalid
//reference of array which size is zero is invalid
typedef A &RA; //C2265
//decltype can generate reference of array which size is zero?
typedef decltype((a)) RA2; //ok: char(&)[]
char *b = "b";
char c[] = "c";
template<typename T>
void f(T const &){}
void f(RA2){}
//reference to zero-sized array is invalid
//reference of array which size is zero is invalid
template<typename T>
void f(T const (&)[]{} //C2265
template<typename T>
struct zero_array_ref{
static T dummy[];
typedef decltype((dummy)) type;
};
//can define, but cannot specify
//can define, but cannot specify
template<typename T>
void f2(typename zero_array_ref<T>::type){}

int main(int argc, char *argv[]){
f(a); //ok: f(RA2) is called
f(b); //ok: f(T const &) is called
f(c); //ok: f(T const &) is called
f(argv[0]); //ok: f(T const &) is called. argv[0] is not char[] but char*
//cannot reduce template arguments against 'T'
//cannot reduce template arguments against T
f2(a); //C2783
}
char a[] = "a";

[Actual Results]:
char (&)[] can only be declared with decltype
'T (&)[]' cannot be declared
char (&)[] can be declared with only decltype
T (&)[] cannot be declared

[Expected Results]:
Both char (&)[] and T (&)[] can be declared without decltype
char (&)[] and T (&)[] can be declared widhout decltype
char (&)[] can only be declared with 'decltype'
T (&)[] cannot be declared
char (&)[] can be declared with only decltype
T (&)[] cannot be declared
Both char (&)[] and T (&)[] can be declared without decltype
char (&)[] and T (&)[] can be declared widhout decltype

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