At most one rest element is allowed in a list or map patternDRT-W1315

The analyzer produces this diagnostic when there's more than one rest pattern in either a list or map pattern. A rest pattern will capture any values unmatched by other subpatterns, making subsequent rest patterns unnecessary because there's nothing left to capture.

Example

The following code produces this diagnostic because there are two rest patterns in the list pattern:

void f(List<int> x) {
  if (x case [0, ..., ...]) {}
}

Common fixes

Remove all but one of the rest patterns:

void f(List<int> x) {
  if (x case [0, ...]) {}
}