class Foo {
public:
private:
    static const int kRows = 4;
    static const int kCols = 4;

    union {
        float m_n[kRows][kCols];
        float m_m[kRows * kCols] = {
            1.0f, 0.0f, 0.0f, 0.0f,
            0.0f, 1.0f, 0.0f, 0.0f,
            0.0f, 0.0f, 1.0f, 0.0f,
            0.0f, 0.0f, 0.0f, 1.0f
        };
    };
};

int main()
{
    Foo myFoo;
    return 0;
}

Compile: g++ -std=c++11 foo.cc

Result:

foo.cc: In constructor ‘constexpr Foo::Foo()’:
foo.cc:1:7: internal compiler error: in cx_check_missing_mem_inits, at cp/semantics.c:6166
 class Foo {
       ^

foo.cc:1:7: internal compiler error: Abort trap: 6
g++: internal compiler error: Abort trap: 6 (program cc1plus)
Abort trap: 6

System:
Darwin Kernel Version 12.2.0: Sat Aug 25 00:48:52 PDT 2012; root:xnu-2050.18.24~1/RELEASE_X86_64 x86_64

GCC build command: ./configure --enable-languages=c,c++

g++ --version
g++ (GCC) 4.8.0 20121020 (experimental)

Additional notes:
By either naming the union, or supplying a default constructor the sample compiles.