Why Does `static_assert` Fail Compilation for an Uncalled Template Function?

Why Does static_assert Fail Compilation for an Uncalled Template Function?

When using template static_assert, it's expected that the assertion fails only when the template function is instantiated. However, in certain cases, like the one raised below, the compilation fails even before the function is called:

template <typename T>
inline T getValue(AnObject&)
{
    static_assert(false , "this function has to be implemented for desired type");
}

Explanation

According to the C standard in [temp.res]/8:

"If no valid specialization can be generated for a template definition, and that template is not instantiated, the template definition is ill-formed, no diagnostic required."

In the provided template, no valid specialization can be generated because the static_assert condition is always false. Therefore, the template definition is ill-formed. Even though it's not instantiated, the compiler may reject it early.

Solution

To resolve this issue, the template can be modified as follows:

template<typename T>
struct foobar : std::false_type
{ };

template <typename T>
inline T getValue(AnObject&)
{
    static_assert( foobar<T>::value , "this function has to be implemented for desired type");
}

This way, the compiler cannot immediately reject the function template since it needs to instantiate the appropriate specialization of foobar before evaluating the static_assert condition. As a result, the compilation error will only occur when the function is actually instantiated and the assertion fails.

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