What Happens to Static Assertions in Untaken `constexpr if` Branches?

Static Assertions in Failed Constexpr if Blocks

Introduction

constexpr if introduced in C 17 allows for more concise and efficient code. However, it also raises questions regarding the handling of static_assert statements in these if blocks.

Failure of Static Assertions in Non-taken Branches

The standard text states that static_assert statements are ill-formed in non-taken branches of constexpr if statements. This applies regardless of whether the condition is dependent or not.

Underlying Rule

This prohibition is based on a well-established rule for templates, which states that a program is ill-formed NDR (no diagnostic required) if no valid specialization can be generated for a template. Static_asserts with nondependent conditions that evaluate to false fall into this category.

Impact on Safety and Usefulness

This limitation puts a significant constraint on the safety and usefulness of constexpr if. Developers must be aware of any potential static_asserts that may be called within the non-taken branch of the if statement.

Example

The following code compiles without warnings but is considered ill-formed:

template< typename T>
constexpr void other_library_foo(){
    static_assert(std::is_same<T,int>::value);
}

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

Scope of the Rule

The prohibition against static_asserts extends to call graphs, so any function called within the non-taken branch that may contain static_asserts is also prohibited.

Conclusion

The failure of static_asserts in non-taken branches of constexpr if statements highlights the need for careful consideration when using them within templates. Developers must ensure that any static_asserts with nondependent conditions that evaluate to false are not attempted in the non-taken branch of the if statement.

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