Hint for missing return when the function is guaranteed to throw

[DartBot]

This issue was originally filed by greg...@gmail.com

---

Ideally the analyzer would not generate a hint for the function below:

//Hint: This function declares the return type of 'int' but does not end with a return statement.

  int foo() {
    bar() => throw "oi";
    bar();
  }

Here's the actual function that I was writing when I noticed this.

  Duration _parseWithSuffix(String s) {
    fail(s) => throw new FormatException('Cannot parse string as duration: "$s".');
    
    parsePrefix(s, [int suffixLen = 1])
      => int.parse(s.substring(0, s.length-suffixLen), onError: fail);
    
    if (s.endsWith('h')) return new Duration(hours: parsePrefix(s));
    if (s.endsWith('m')) return new Duration(minutes: parsePrefix(s));
    if (s.endsWith('s')) return new Duration(seconds: parsePrefix(s));
    if (s.endsWith('ms')) return new Duration(milliseconds: parsePrefix(s, 2));
    if (s.endsWith('us')) return new Duration(microseconds: parsePrefix(s, 2));
    
    fail(s);
  }

[kasperl]

Added Area-Analyzer, Triaged labels.

[bwilkerson]

While I agree that this would be nice, and potentially possible when dealing with locally defined functions, the full solution (being able to trace all possible execution paths to see whether they all throw an exception) is a bit more than we have resources to do at the moment.

I will note that as a workaround (albeit an ugly workaround) the analyzer recognizes a throw as an exit from the method, so if you inline final invocation of the local function 'fail' the hint will not be generated.

---

Removed Priority-Unassigned label.
Added Priority-Medium label.

[nex3]

Would it be possible to add an @alwaysThrows annotation that the analyzer understands, sort of like @deprecated? This would be really useful for stuff like the test package's fail() method.

[sethladd]

Another case for Analyzer plugins! :)

[srawlins]

CC #17999

[srawlins]

@alwaysThrows has been implemented.