mangling for instantiation-dependent non-type template parameter types #20

The mangling for a function template does not include the instantiation-dependent portions of non-type template parameters (including such things transitively within template template parameters). This is becoming increasingly important as people try to write things like:

template<typename T, std::void_t<typename T::x>* = nullptr> void func() {}
template<typename T, std::void_t<typename T::y>* = nullptr> void func() {}

(std::void_t<T> is void for all T. It's not obvious whether it's supposed to be a dependent type, but the above cases are at least instantiation-dependent types.)

For a type T providing both a nested x and a nested y, we will mangle instantiations of the two possible func<T>s the same, despite them being distinct templates.

Including the (pre-substitution) types of non-type template parameters in the mangling (if they're instantiation-dependent) seems like the obvious fix, but it would likely result in an ABI break for a significant amount of existing code.

We should probably at least fix this for ABI v2.