Why Are Static Assertions in the False Branch of a Constexpr If Statement Ill-Formed?

Static_Asserts in Failed Constexpr If Blocks: An Analysis

In C 17, constexpr if statements provide a means to conditionally execute code at compile time. However, the behavior of static_asserts within the false branch of such statements has raised concerns.

The issue arises from the rule that disallows the instantiation of a template containing a static_assert whose condition is non-dependent and evaluates to false ([temp.res]/8). This rule ensures type safety by preventing undefined behavior in subsequent template expansions.

In the context of a constexpr if statement, this means that static_asserts in the false branch are inherently ill-formed if their condition is non-dependent. This is because the compiler cannot determine at compile time whether any valid specialization can be generated for the template containing the static_assert.

For example, the following code is ill-formed:

void f() {
  if constexpr (false)
    static_assert(false);   // ill-formed
}

Similarly, static_asserts within constexpr template functions in the false branch are also ill-formed:

template<class T>
void g() {
  if constexpr (false)
    static_assert(false);   // ill-formed
}

This rule extends to indirect calls to constexpr functions or template functions that call static_asserts. Any such calls within the false branch of a constexpr if statement are forbidden, even if they themselves do not contain static_asserts.

This prohibition limits the usefulness of constexpr if statements, as it requires developers to meticulously scrutinize code to ensure the absence of static_asserts in the false branch. However, static_asserts with dependent conditions that can evaluate to true for at least one type are still permissible within constexpr if statements.

In conclusion, the ill-formed nature of static_asserts in failed constexpr if blocks is a consequence of the general rule that prohibits templates from containing static_asserts with non-dependent conditions that evaluate to false. It is crucial for developers to adhere to this rule to avoid undefined behavior and maintain type safety in their code.

The above is the detailed content of Why Are Static Assertions in the False Branch of a Constexpr If Statement Ill-Formed?. For more information, please follow other related articles on the PHP Chinese website!