Difference between 12.4 & gcc/clang w/regard to anonymous structs in unions

Question

I'm actually reporting two differences, but subjects can only be so long.
union Blah {
  struct {
    unsigned char x[8];
  };  // Sun Studio error: A declaration does not specify a tag or an identifier
      // note: -features=extensions is required to get this to compile
  unsigned int y[2];
  void SwapBytes() {
    for( int ii = 0; /*...*/ ) {
      x[ii] = /*...*/;  // Sun Studio error: x is not defined
int main( int argc, char *argv[] ) {
  Blah b = { 0xDEADBEEF, 0xBADBABE0 };  // clang/gcc warn that this is a narrowing conversion because the
                                        // default behavior for brace initialization is to initialize according
                                        // to the first type found in the union.
                                        // Sun Studio only warns with +w2, and only reports the more opaque msg:
                                        //   Warning: partially bracketed initialization for b
  return 0;
There are two problems reported here:
gcc/clang allow the anonymous struct -- it's useful for preventing access to the array of bytes but allowing union member functions access to it
difference in when/why the bracketed initialization produces a warning
-Brian

Answer 1

First, a correction. The Studio 12.4 compiler allows anonymous structs by default, unlike earlier compilers. Apparently we decided to step to the other side of this line. :-) The compiler does issue a warning for the non-standard code.
The narrowing conversions in the example
Blah b = { 0xDEADBEEF, 0xBADBABE0 };
are errors in C++11, not warnings. In the C++11 standard, refer to section 8.5.4 List-initialization. This rule is a change from the previous C++ standard, one of a very small number of cases where valid code becomes invalid.
Not all narrowing conversions are errors, only narrowing conversions in initializer-lists. You can make the code valid by inserting an explicit cast.