Does `static_assert` Behavior Vary in Template Instantiation Between GCC and Clang?

Assessment of static_assert Behavior in Template Instantiation

In this code, a static_assert is employed to validate the value of the non-type template parameter answer. However, the behavior of this assertion varies between gcc and clang:

<code class="cpp">template <int answer> struct Hitchhiker {
  static_assert(sizeof(answer) != sizeof(answer), "Invalid answer");
};

template <> struct Hitchhiker<42> {};</code>

gcc's Behavior:

gcc only triggers the assertion when instantiating Hitchhiker with a parameter other than 42.

clang's Behavior:

clang raises the assertion error even if the template is not explicitly instantiated.

Standard Interpretation:

The C standard states that if a template has no valid specializations and is not instantiated, the template is considered ill-formed. This means that no diagnostic is required.

Analysis:

Both compilers are correct in their behavior. Clang chooses to provide a diagnostic even though the standard does not require it.

Alternative Approach:

To only permit an answer of 42, the general template can be omitted, and the specialized template defined as follows:

<code class="cpp">template <> struct Hitchhiker<42> {};</code>

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