unnecessary_set_literal

Braces unnecessarily wrap this expression in a set literal.

The analyzer produces this diagnostic when a function that has a return type of void, Future<void>, or FutureOr<void> uses an expression function body (=>) and the returned value is a literal set containing a single element.

Although the language allows it, returning a value from a void function isn't useful because it can't be used at the call site. In this particular case the return is often due to a misunderstanding about the syntax. The braces aren't necessary and can be removed.

The following code produces this diagnostic because the closure being passed to g has a return type of void, but is returning a set:

void f() {
  g(() => {1});
}

void g(void Function() p) {}

Remove the braces from around the value:

void f() {
  g(() => 1);
}

void g(void Function() p) {}

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