ambiguous_extension_member_access

A member named '{0}' is defined in '{1}' and '{2}', and neither is more specific.

A member named '{0}' is defined in {1}, and none are more specific.

When code refers to a member of an object (for example, o.m() or o.m or o[i]) where the static type of o doesn't declare the member (m or [], for example), then the analyzer tries to find the member in an extension. For example, if the member is m, then the analyzer looks for extensions that declare a member named m and have an extended type that the static type of o can be assigned to. When there's more than one such extension in scope, the extension whose extended type is most specific is selected.

The analyzer produces this diagnostic when none of the extensions has an extended type that's more specific than the extended types of all of the other extensions, making the reference to the member ambiguous.

The following code produces this diagnostic because there's no way to choose between the member in E1 and the member in E2:

extension E1 on String {
  int get charCount => 1;
}
​
extension E2 on String {
  int get charCount => 2;
}
​
void f(String s) {
  print(s.charCount);
}

If you don't need both extensions, then you can delete or hide one of them.

If you need both, then explicitly select the one you want to use by using an extension override:

extension E1 on String {
  int get charCount => length;
}
​
extension E2 on String {
  int get charCount => length;
}
​
void f(String s) {
  print(E2(s).charCount);
}