Unreachable code should be detected by default without `--warn-unreachable` flag

[hyperkai]

With --warn-unreachable flag, unreachable code can be detected as shown below:

def func() -> int:
    return 100
    print('Unreachable')
# error: Statement is unreachable

But without --warn-unreachable flag, unreachable code cannot be detected as shown below so unreachable code should be detected by default without --warn-unreachable flag:

def func() -> int:
    return 100
    print('Unreachable')
# No error

Actually, Java detects unreachable code by default as shown below:

class Main {
    public static void main(String[] args) {
        System.out.println(func());
    }
    
    public static int func() {
        return 100;
        System.out.println("Unreachable"); // error: unreachable statement
    }
}

And, Go also detects unreachable code by default as shown below:

package main

import "fmt"

func main() {
	fmt.Println(myfunc())
}

func myfunc() int {
	return 100
	fmt.Println("Unreachable") // missing return
}

[A5rocks]

See #18078 for the enabling it by default with --strict. Probably it's a bad idea to enable it by default for non-strict! Java and Go have the advantages that the types must be followed... mypy doesn't have that. --warn-unreachable is an especially egregious false-positive in this case (that the type hints aren't followed somewhere in the program) so IMO it's bad UX to enable it by default.

I won't close this though.