context-related optional chaining support in paths

[lifeart]

related issue: #50

for simple usage, like:

{{if this.foo.bar "a" "b"}}

{{#if this.foo.bar}}
A
{{/if}}

{{unless this.foo.bar "a" "b"}}

{{#unless this.foo.bar}}
B
{{/unless}}

{{#each this.foo.bar as |item|}}

{{/each}}

this.foo.bar path should be translated into this?.foo?.bar;

for complex usage, like:

<div {{on "click" this.foo.bar.onClick}} />

this.foo.bar.onClick should not be translated as optional chained

[boris-petrov]

A couple of comments:

1. I'm not sure this? is sensible. When can this be undefined? Apart from template-only components, but not sure if that can't be handled in another way.
2. Also, this.foo.bar should be treated as this.foo?.bar only if foo is nullable - e.g. public foo?: string. Otherwise it should be treated as this.foo.bar.
3. For the on "click" case - again, the whole path should be treated the same way as point 2. It would type-check only if none of the parts are nullable (or have been check before that with an if statement) - just like in TypeScript.

[lifeart]

agree on 1, for 2 should we just make tail optional?

for 3 - I need to play more with if convertation, currently it's not represented as logical part, only as type (currently we can't chain type checks)

[boris-petrov]

Not sure what you mean by tail for number 2? You mean bar?

I think the whole thing should be mentally very simple - you have a path like this.foo.bar in HBS. We want that to type-check no matter what the path is (because that's HBS' semantics - the path will not blow up at runtime ever - but it might return undefined if any of the parts of the path returns undefined). So if foo is non-nullable (say this is { foo: { bar: string } }) then that's OK and can be translated directly to this.foo.bar. If foo is nullable (say this is { foo?: { bar: string } }) then to make this chain type-check in TypeScript we must optional-chain foo - this.foo?.bar. This would make TS happy and the whole chain would be typed as string | undefined which is exactly what the HBS type should be.

This line of thought will work for all cases we discussed in issues here and in Discord - for each, on, if, etc.