Why Does `static_assert` Behavior Vary Between GCC and Clang When Dependent on a Non-Type Template Parameter?

static_assert Dependent on Non-Type Template Parameter: Varying Behavior in GCC and Clang

In C , the static_assert macro is used to verify compile-time conditions. When the condition fails, the program is terminated with an error message.

Consider the following code:

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

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

This code attempts to disable general template instantiation with static_assert. However, its behavior differs between compilers:

• GCC (4.9.2): Throws an assert error only when instantiating Hitchhiker with a parameter other than 42.
• Clang (3.50): Throws the error even when the template is not instantiated.

The Standard's Perspective

The C standard dictates that a template is ill-formed if no valid specialization can be generated and it is not instantiated. In this case, there is no valid specialization for the Hitchhiker template, so it is technically ill-formed.

However, the standard does not require a diagnostic to be issued in such cases.

Compiler Implementations

GCC chooses to adhere to the standard strictly, issuing no diagnostic when the template is not instantiated (because it is already ill-formed).

Clang, on the other hand, provides a more user-friendly experience by issuing a diagnostic even when the template is not instantiated. This helps developers identify potential issues earlier in the compilation process.

Best Practice

To avoid this discrepancy, it is recommended to explicitly specialize the template for the desired parameter value instead of using static_assert to disable general template instantiation. This approach ensures consistent behavior across compilers.

For example:

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

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