Using C++14. Why will this compile:
template
constexpr bool foo()
{
std::array arr;
return true;
}
but
They don't. Your test is flawed.
The problem is not detected until you actually attempt to instantiate the broken function template.
§7.1.5 [dcl.constexpr]/p6:
If the instantiated template specialization of a
constexpr
function template or member function of a class template would fail to satisfy the requirements for aconstexpr
function orconstexpr
constructor, that specialization is still aconstexpr
function orconstexpr
constructor, even though a call to such a function cannot appear in a constant expression. If no specialization of the template would satisfy the requirements for aconstexpr
function orconstexpr
constructor when considered as a non-template function or constructor, the template is ill-formed; no diagnostic required.
It is valid for constexpr
function templates to have some specializations that do not satisfy the constexpr
requirements, and it is valid to use those specializations as long as they are not in a context that requires a constant expression.
It isn't valid, however, if no specialization of the template could satisfy constexpr
requirements. Since in the general case it is impossible to determine whether all possible instantiations of a function template will fail to satisfy the constexpr
requirements,the standard doesn't require a diagnostic. Hence, your code is ill-formed with no diagnostic required - the compiler can, but is not required to, report an error.