G++ accepts definitions of C flexible array members as an extension.  However, since C specifies that a flexible array member has an incomplete type, such a member is not a valid operand in expressions that require an object to have a complete type.  An example is the sizeof expression.  Since flexible array members are provided for compatibility with C, they should be subject to the same requirements and restrictions.

$ cat z.cpp && /build/gcc-trunk-svn/gcc/xgcc -B /build/gcc-trunk-svn/gcc -Wall -Wextra -Wpedantic -xc++ z.cpp
struct S {
    int n;
    int a[];
} s;

extern int a[];

enum E {
    e = sizeof s.a,   // rejected in C, accepted in C++
    f = sizeof a      // rejected in both C and C++
};

z.cpp:3:11: warning: ISO C++ forbids zero-size array ‘a’ [-Wpedantic]
     int a[];
           ^

z.cpp:10:16: error: invalid application of ‘sizeof’ to incomplete type ‘int []’
     f = sizeof a      // rejected in both C and C++
                ^