Deprecation Guide for Invoking Helpers Without Arguments and Parentheses In Named Argument Positions

With contextual helpers arriving in Ember, helpers, modifiers and components can increasingly be thought of as first-class variables that can be passed around.

Invoking a helper without arguments or parentheses in named argument positions can be ambigious and conflicts with this mental model:

<SomeComponent @arg={{someHelper}} />

In this case, it's ambigious between passing someHelper as a value to the component to be invoked later or invoking the helper with no arguments and passing the result into the component.

The current behavior is to invoke the helper with no arguments and pass in the result, but this is counterintuitive in light of the broader "helper as a value" mental model. Therefore, this invocation style is deprecated in favor of explicitly invoking the helper with parentheses:

<SomeComponent @arg={{(someHelper)}} />

Note that this is only required in this specific scenario, where:

1. This is not in a strict mode context, AND
2. someHelper is a global helper, i.e. not this.someHelper, @someHelper or a local variable ({{#... as |someHelper|}}), AND
3. No arguments are provided to the helper, AND
4. It's in an angle bracket component invocation's named argument position, i.e. not <div id={{someHelper}}> or <Foo bar={{someHelper}}> or {{foo bar=(someHelper)}}, AND
5. Not parenthesized, i.e. not @foo={{(helper)}}, AND
6. Not interpolated, i.e. not @foo="{{helper}}".

In pratice, this is quite rare, as it is rather uncommon for helpers to not take any arguments.