The null-assert pattern will have no effect because the matched type isn't nullableDRT-W1683

The analyzer produces this diagnostic when a null-assert pattern is used to match a value that isn't nullable.

Example

The following code produces this diagnostic because the variable x isn't nullable:

void f(int x) {
  if (x case var a! when a > 0) {}
}

Common fixes

Remove the null-assert pattern:

void f(int x) {
  if (x case var a when a > 0) {}
}