Switch expression doesn't actually seem to be an expression in Dart 3.0

In Dart 3.0, switch expression is finally introduced today.

According to the official documentation,

> A switch expression is similar to a switch statement, but you can use them anywhere you can use an expression.

However, the code below doesn't compile (DartPad):

enum Color { red, blue, green }

void main() {
  final color = Color.red;
  switch (color) {
    Color.red => "red",
    Color.blue => "blue",
    Color.green => "green",
  };
}

though these code compile:

enum Color { red, blue, green }

void main() {
  final color = Color.red;
  //binds to placeholder
  final _ = switch (color) {
    Color.red => "red",
    Color.blue => "blue",
    Color.green => "green",
  };
}

void main() {
  3; //expression statement
}

Why?

(This question is about the language itself rather than practical applications. So the codes above are minimal but aren't useful. Thus I added #language-lawyer tag.)

Note, in Rust, the equivalent code compiles:

enum Color {
    Red,
    Blue,
    Green,
}

fn main() {
    let color = Color::Red;
    match (color) {
        Color::Red => "red",
        Color::Blue => "blue",
        Color::Green => "green",
    };
}

Switch expression at a start of an expression statement is explicitly forbidden by the language spec. Here's the relevant snippet from the spec:

Expression statement ambiguity

Thanks to expression statements, a switch expression could appear in the same
position as a switch statement. This isn't technically ambiguous, but requires
unbounded lookahead to read past the value expression to the first case in
order to tell if a switch in statement position is a statement or expression.

main() {
  switch (some(extremely, long, expression, here)) {
    _ => expression()
  };

  switch (some(extremely, long, expression, here)) {
    case _: statement();
  }
}

To avoid that, we disallow a switch expression from appearing at the beginning
of an expression statement. This is similar to existing restrictions on map
literals appearing in expression statements. In the rare case where a user
really wants one there, they can parenthesize it.

So the fix would be parenthesizing the expression:

enum Color { red, blue, green }

void main() {
  final color = Color.red;
  (switch (color) {
    Color.red => "red",
    Color.blue => "blue",
    Color.green => "green",
  });
}

ref: dart-lang/language #3061.