Why are `static_assert`s in the `false` branch of a `constexpr if` ill-formed?

constexpr if with Failed Static_Assert in the False Branch

Question:

Despite the proposal's lack of explicit mention of static_assert, why is its use within the non-taken branch of a constexpr if statement considered ill-formed?

Answer:

According to the rule for templates ([temp.res]/8), a program is ill-formed NDR (no diagnostic required) if:

• No valid specialization can be generated for a template, or a substatement of a constexpr if statement within a template.
• The template is not instantiated.

In the case of a static_assert with a nondependent condition that evaluates to false, no valid specialization can be generated for the template containing the static_assert. Therefore, the program containing such a statement is ill-formed.

Example:

`
void f() {
if constexpr (false)

static_assert(false);   // ill-formed

}
`

In this example, the static_assert in the non-taken branch of the constexpr if statement makes the entire function ill-formed, as no valid specialization can be generated for a template containing the static_assert.

However, this rule does not affect static_asserts with a dependent condition that can evaluate to true for at least one type.

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