Inconsistent Behavior of static_assert with Non-Type Template Parameters Across Compilers

In C , a static_assert can be used to verify conditions at compile time. However, a recent observation has revealed a disparity in the behavior of static_assert when used in conjunction with non-type template parameters across different compilers.

Specifically, the following code snippet:

template 
struct Hitchhiker {
  static_assert(sizeof(answer) != sizeof(answer), "Invalid answer");
};

template 
struct Hitchhiker {};

behaves differently on Clang and GCC when attempting to disable general template instantiation using static_assert. Clang triggers the assert error even when the template is not instantiated, while GCC only raises the error upon instantiation with a parameter value other than 42.

To understand the discrepancy, let's explore the relevant section of the C standard ([temp.res]/8):

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

According to this rule, the primary template Hitchhiker is ill-formed since no valid specialization can be generated. As such, it is not required to issue a diagnostic. However, Clang chooses to provide a diagnostic despite the lack of requirement.

If the intention is to restrict the instantiation to only 42, the recommended approach is to omit the general template definition as follows:

template 
struct Hitchhiker {};