Analyzer allows expression with type void as expression switch case expression.

[lrhn]

See dart-lang/language#2907 (comment) for context.

Example:

void main() => switch (1) {_ => print("ok")};

The CFE gives the error:

lib/main.dart:1:33:
Error: This expression has type 'void' and can't be used.
void main() => switch (1) {_ => print("ok")};
                                ^
Error: Compilation failed.

That is correct, because expressions of type void are only allowed in specific positions, and the expression of an expression switch case isn't on the list.
It didn't exist when the list was made, but we also haven't added the new position to that list.

The analyzer does not give the required error.

(We can choose to allow an expression switch case expression to have type void when the expression switch itself is allowed to have type void. We can't just say that it's OK for the case expression to have type void, because that void might get lost in a LUB with another branch's type. Unless a LUB containing at least one void always ends up as void. I don't know if it does, even when LUB'ed with, say, FutureOr<void>.)

[scheglov]

https://dart-review.googlesource.com/c/sdk/+/298400

[leafpetersen]

Unless a LUB containing at least one void always ends up as void. I don't know if it does, even when LUB'ed with, say, FutureOr<void>.

It does. See here.