Why is non-type template parameter expression handling inconsistent across compilers?

Here is something I observed across various compilers. It seems there are compiler bugs.

template <int I>
struct X
{ };

int main(void)
{
  X<(16 > 1)> a;       // Works on vc9, works on g++ 4.1.2, works on Comeau 4.3.10.1
  X<(int(16) > 1)> b;  // Works on vc9, works on g++ 4.1.2, works on Comeau 4.3.10.1
  X<(16 >> 1)> c;      // Works on vc9, works on g++ 4.1.2, works on Comeau 4.3.10.1
  X<(int(16) >> 1)> d; // Fails on vc9, works on g++ 4.1.2, works on Comeau 4.3.10.1

  X<16 > 1> e;         // Fails on vc9, works on g++ 4.1.2, fails on Comeau 4.3.10.1
  X<int(16) > 1> f;    // Fails on vc9, fails on g++ 4.1.2, fails on Comeau 4.3.10.1
  X<16 >> 1> g;        // Fails on vc9, works on g++ 4.1.2, fails on Comeau 4.3.10.1
  X<int(16) >> 1> h;   // Fails on vc9, works on g++ 4.1.2, fails on Comeau 4.3.10.1
 }

Why is that inconsistency? What is allowed/disallowed by the standard? Such behavior is also responsible for syntax error while using BOOST_AUTO on vc9. It appears to me that Comeau is doing the right job by rejecting all the expressions without parenthesis.