The following program defines two structs whose only member is a flexible array.  Such types are invalid and should be rejected.  G++ diagnoses the one defined in the named struct but fails to diagnose the one in the unnamed struct.

$ cat t.C && /build/gcc-trunk-svn/gcc/xgcc -B /build/gcc-trunk-svn/gcc -S -Wall -Wextra -Wpedantic t.C
struct A {
  int a[];
};

typedef struct {
  int a[];
} B;
t.C:2:9: error: flexible array member ‘A::a’ in an otherwise empty ‘struct A’
   int a[];
         ^
t.C:1:8: note: in the definition of ‘struct A’
 struct A {
        ^