How Does a Failed `static_assert` in an Untaken `if constexpr` Block Affect Program Validity?

How does a failed static_assert impact an if constexpr (false) block?

A static_assert within a non-taken branch of an if constexpr statement is deemed ill-formed, with no diagnostic required. This consequence stems from the rule in [temp.res]/8, which deems a program ill-formed if no valid specialization can be generated for a template or a substatement within a constexpr if statement.

In the case of static_assert, if the condition is nondependent and evaluates to false, no valid specialization can be generated for the template containing the assertion. This makes the program ill-formed, even if the branch is not taken.

However, static_asserts with a dependent condition remain unaffected. If the condition can evaluate to true for at least one type, the template remains valid.

Example

Consider the following code:

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>(); 
}

int main(){
    g<float>();
    g<int>();
}

Despite the if constexpr condition being false, the code is still ill-formed because the static_assert in other_library_foo contains a nondependent condition that evaluates to false.

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