Warn about incorrect use of `default` in ternary operator

[bash]

I just ran into a footgun with our implicit option constructor:

The following snippet does not return an empty option but a some-option with a default-initialized tuple inside:

Option<(string, int)> DefaultUsage(bool cond) => cond ? ("hello world", 42) : default;

This of course applies to all value types:

Option<int> DefaultUsage() => false ? 42 : default; // returns Some(0), not None.

for reference types, we get at least a nullable warning:

Option<string> DefaultUsage() => false ? "hello world" : default;
//                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
// warning CS8604: Possible null reference argument for parameter 'item'
// in 'Option<string>.implicit operator Option<string>(string item)'.

We should emit a warning in these cases:

• default inside ternary inside implicit conversion to Option
• default inside switch expression inside implicit conversion to Option
• others?

This is of course perfectly fine and returns None as expected:

Option<(string, int)> DefaultUsageFine(bool cond)
{
    if (cond) { return ("hello world", 42); }
    else { return default; }
}

[FreeApophis]

That's a fun one, never run into this.

The issue would also be resolved if we would to #760 and remove the implicit operator.