When can outer braces be omitted in an initializer list?

I've got error C2078 in VC2010 when compiling the code below.

struct A
  {
  int foo;
  double bar;
  };

std::array<A, 2> a1 = 
  // error C2078: too many initializers
  {
    {0, 0.1},
    {2, 3.4}
  };

// OK
std::array<double, 2> a2 = {0.1, 2.3};

I found out that the correct syntax for a1 is

std::array<A, 2> a1 = 
  {{
    {0, 0.1},
    {2, 3.4}
  }};

The question is: why extra braces are required for a1 but not required for a2?

Update

The question seems to be not specific to std::array. Some examples:

struct B
  {
  int foo[2];
  };

// OK
B meow1 = {1,2};
B bark1 = {{1,2}};

struct C
  {
  struct 
    { 
    int a, b; 
    } foo;
  };

// OK
C meow2 = {1,2};
C bark2 = {{1,2}};

struct D
  {
  struct 
    { 
    int a, b; 
    } foo[2];
  };

D meow3 = {{1,2},{3,4}};  // error C2078: too many initializers
D bark3 = {{{1,2},{3,4}}};

I still don't see why struct D gives the error but B and C don't.