The following program does not compile with clang r193986:

template <typename T>
constexpr T PI = T(3.14);

template <typename T>
constexpr T TAU = 2 * PI<T>;

// A: Without this line using TAU below results in a compile error
//constexpr double HALF_PI = PI<double> / 2;


int main(int argc, char**)
{
    return TAU<double> * argc;
}


This results in:

 ~/LLVM/build/Release+Asserts/bin/clang -c -std=c++1y clang.cpp   
clang.cpp:5:23: error: constexpr variable 'TAU<double>' must be initialized by a constant expression
constexpr T TAU = 2 * PI<T>;
                  ~~~~^~
clang.cpp:13:12: note: in instantiation of variable template specialization 'TAU<double>' requested here
    return TAU<double> * argc;
           ^
1 error generated.


After uncommenting the line below "A", the code compiles successfully.

It seems that a constexpr variable template cannot be used in the definition of another constexpr variable template when it is not used in the definition of a non-template constexpr variable as well.