Why Do Static Assertions Fail in C 17\'s `constexpr if (false)` Blocks?

Failed Static_Assert Behavior in Constexpr If (False) Blocks

The introduction of constexpr if in C 17 has raised concerns about the behavior of static_asserts within the non-taken branch of these statements.

According to the C standard, when a constexpr if statement appears in a templated entity, discarded statements within the non-taken branch are not instantiated. This rule prohibits the execution of static_asserts with nondependent false conditions during template instantiation, effectively deeming the program ill-formed.

This behavior stems from a more general rule that applies to all templates: if no valid specialization can be generated for a template with a substatement in a constexpr if that evaluates to false, the program is considered ill-formed, regardless of whether the template is instantiated.

For example, the following code is ill-formed, though it compiles without warning using recent versions of Clang:

template
constexpr void other_library_foo(){
    static_assert(std::is_same<t>::value);
}

template<class t>
void g() {
  if constexpr (false)
    other_library_foo<t>(); 
}</t></class></t>

This behavior extends to indirect calls to static_asserts. If a constexpr (template) function contains a static_assert with a nondependent false condition, it cannot be called within the non-taken branch of a constexpr if statement, even within a separate template.

Implications and Precautions

This limitation on static_asserts within constexpr if (false) blocks can impact the safety and usefulness of constexpr if as a replacement for SFINAE. Developers must be aware of any potential use of static_asserts in functions or libraries that may be indirectly invoked within the non-taken branch of such statements.

As a result, it is advisable to avoid using static_asserts in code that may be executed as part of a constexpr if (false) block. This practice ensures that programs remain well-formed and avoids potential compiler warnings or errors.

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