Why Does a Static Assertion Fail Compilation Despite a Null Function Invocation?

Static Assertion Fails Compilation Despite Null Function Invocation

Using g 4.6.3 with the c 0x flag, developers have encountered an unexpected compilation error:

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

Results in:

static_assertion failed "this function has to be implemented for the desired type"

This error occurs despite the function not being invoked anywhere in the code.

Explanation

According to the C standard [temp.res]/8, if no valid specialization can be generated for a template definition that is not instantiated, the template is ill-formed. While the compiler is not obligated to diagnose this error, it is permitted to reject the template.

Resolution

One approach to resolve this issue is to use a type trait to guard the static assertion:

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");
}

With this modification, the compiler cannot reject the template immediately as it needs to instantiate the relevant specialization of foobar to determine the value of the static assertion, which will still fail as intended.

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