Fix/Improve Nullability Analysis in Loops

[TonyValenti]

In the example below, ret should be definitely assigned since the loop is guaranteed to run.

#nullable enable

using System;

namespace ConsoleApp23 {
    class Program {
        static void Main(string[] args) {
            TestObject();
        }

        public static object TestObject() {
            var ret = default(object?);

            for (int i = 0; i < 1; i++) {
                ret = new();
            }
            return ret;
        }

    }
}

[huoyaoyuan]

since the loop is guaranteed to run.

Analyzing arbitrary loop is impossible. The current behavior only handles constant true and false, which is fine.

If we have something like "range based loops", analysis for constant ranges is good. Treating for loop as range based loop is quite complex, since small modification can make the loop no longer recognized.

[HaloFour]

The definite assignment flow analysis in the compiler doesn't handle this kind of case either. If you don't give ret a default value on declaration it will be a compiler error to attempt to return it from the method.