This is talking about a well-established rule for templates – the same rule that allows compilers to diagnose template<class> void f() { return 1; }. [temp.res]/8 with the new change bolded:
The program is ill-formed, no diagnostic required, if:
- no valid specialization can be generated for a template or a substatement
of aconstexpr ifstatement ([stmt.if]) within a
template and the template is not instantiated, or- […]
No valid specialization can be generated for a template containing static_assert whose condition is nondependent and evaluates to false, so the program is ill-formed NDR.
static_asserts with a dependent condition that can evaluate to true for at least one type are not affected.