[analyzer] suggest truncate-divide when double-from-divide is used

[srawlins]

Forked from dart-lang/linter#3930 (comment)

@rakudrama suggests a new error message for code like this:

String suffix(String s) => s.substring(s.length / 2);

Currently this code produces an error, argument_type_not_assignable: "The argument type 'double' can't be assigned to the parameter type 'int'." But it could suggest using /~ instead of /. Then users would be less likely to write (s.length / 2).toInt(), and we could retire the division_optimization hint (which fires for code like (s.length / 2).toInt().

However, to make a sufficiently complete replacement, we'd have to catch more than argument_type_not_assignable. I can think of the following other cases:

// return_of_invalid_type
int f2(String s) => s.length / 2;

// invalid_assignment
int x = "hello".length / 2;

// list_element_type_not_assignable
List<int> f2(String s) => [7, s.length / 2];

I'm sure there are more.

Also, is it sufficient to catch x / y as int? Or would we want to go deeper and catch, for example, (x / y) + 1?

CC @rakudrama @bwilkerson

[bwilkerson]

I suspect that the rule we'd want here is to report this situation when an expression has type double when int is required and the expression is a binary expression using / (and the operands are a combination of double or int that would be valid for ~/).

It's an interesting question as to whether we'd want to catch cases where a subexpression of that form caused the problem. I'd want to understand better how complex the logic would be and compare that to the value to users (cost/benefit analysis).

The cast issue seems different in some ways. I just tried this:

void main() {
  f(12.0, 3);
}

void f(double x, int y) {
  print((x / y) as int);
}

which throws an exception. I guess I expected to be told that the cast would always fail, but I wasn't. Maybe that's a lint that I don't have enabled?

[rakudrama]

With web numbers, where all num are double and some double are int, (x / y) as int passes when y divides x exactly.