Spurious error on unevaluated use of undefined constexpr function

The following code results in an error because the constexpr function a_ is not defined:

constexpr int a_();
char c_[ 2 + (0 && a_()) ];

a_() is unevaluated, and a pedantic reading of the standard (5.19.2) allows the use of an undefined constexpr function when it is called in a constant expression and is unevaluated.