c++ - Why do templates allow constexpr function members with non-constexpr constructors? -

c++ - Why do templates allow constexpr function members with non-constexpr constructors? -

using c++14. why compile:

template<unsigned n> constexpr bool foo() { std::array<char, n> arr; homecoming true; }

but not this?

constexpr bool foo() { std::array<char, 10> arr; // non-constexpr constructor 'array' cannot used in constant look homecoming true; }

§7.1.5 [dcl.constexpr]/p6:

if instantiated template specialization of constexpr function template or fellow member function of class template fail satisfy requirements constexpr function or constexpr constructor, specialization still constexpr function or constexpr constructor, though phone call such function cannot appear in constant expression. if no specialization of template satisfy requirements constexpr function or constexpr constructor when considered non-template function or constructor, template ill-formed; no diagnostic required.

it valid constexpr function templates have specializations not satisfy constexpr requirements, , valid utilize specializations long not in context requires constant expression.

it isn't valid, however, if no specialization of template satisfy constexpr requirements. since in general case impossible determine whether possible instantiations of function template fail satisfy constexpr requirements,the standard doesn't require diagnostic. hence, code ill-formed no diagnostic required - compiler can, not required to, study error.

c++ templates c++14 constexpr