Example Named Yields Result, this is would be the result of AST walker transformation, but can be wired manually.

components_faked-yields_component.js

import Ember from 'ember';

export default Ember.Component.extend({
  header: { title: 'HEADER' },
  footer: { title: 'FOOTER' },
  body: { title: 'BODY' },
  slots: null,

  _registerSlot(name) {
    this.slots.set(name, true);
  },

  init() {
    this._super();
    this.slots = this.slots || Ember.Object.create({});
  }


});

components_faked-yields_template.hbs

<vbox>
  {{#if slots.header}}
    <box fit>
      {{yield (hash name='header' register=(action _registerSlot) params=(arr header 'hello' 'world'))}}
    </box>
  {{/if}}

  <hbox>
    {{yield (hash name='main' register=(action _registerSlot) params=(arr body 'goodnight' 'moon'))}}
  </hbox>

  {{#if slots.footer}}
    <box fit>
      {{yield (hash name='footer' register=(action _registerSlot) params=(arr footer 'awesome' 'you'))}}
    </box>
  {{/if}}
</vbox>

components_slot-proxy_component.js

import Ember from 'ember';

const {
  assert,
  Component,
  computed
  } = Ember;

const component = Component.extend({
  tagName: '',

  name: '',

  // TODO error if slot length is greater than 6
  slot: null,
  register: null,

  shouldDisplay: computed('name', 'slot.name', function() {
    return this.get('name') === this.get('slot.name');
  }),

  p1: computed('slot.params.[]', function() {
    return this.get('slot.params').objectAt(0);
  }),

  p2: computed('slot.params.[]', function() {
    return this.get('slot.params').objectAt(1);
  }),

  p3: computed('slot.params.[]', function() {
    return this.get('slot.params').objectAt(2);
  }),

  p4: computed('slot.params.[]', function() {
    return this.get('slot.params').objectAt(3);
  }),

  p5: computed('slot.params.[]', function() {
    return this.get('slot.params').objectAt(4);
  }),

  init() {
    this._super();
    assert("You must include a name for your block", this.name);
    this.slot.register(this.name);
  }
});

component.reopenClass({
  positionalParams: ['slot', 'name']
});

export default component;

components_slot-proxy_template.hbs

{{#if shouldDisplay}}
  {{yield p1 p2 p3 p4 p5}}
{{/if}}

helpers_arr.js

import Ember from 'ember';

export function arr(params/*, hash*/) {
  return Ember.A(params);
}

export default Ember.Helper.helper(arr);

routes_example-usage_template.hbs

<box>
        {{#faked-yields as |slot|}}
          {{#slot-proxy slot "main" as |info w1 w2|}}
            I am the content {{info.title}} | {{w1}} {{w2}}
          {{/slot-proxy}}
        {{/faked-yields}}
      </box>
      <box>
        {{#faked-yields as |slot|}}
          {{#slot-proxy slot "header" as |info w1 w2|}}
            I am the content {{info.title}} | {{w1}} {{w2}}
          {{/slot-proxy}}
          {{#slot-proxy slot "main" as |info w1 w2|}}
            I am the content {{info.title}} | {{w1}} {{w2}}
          {{/slot-proxy}}
          {{#slot-proxy slot "footer" as |info w1 w2|}}
            I am the content {{info.title}} | {{w1}} {{w2}}
          {{/slot-proxy}}
        {{/faked-yields}}
      </box>

• faked-yields is an example component utilizing slots
• slot-proxy is how we make this work, but comes with a perf overhead since all yields are now wrapped with an extra component and layer
• arr is just a helper to make it easy to pass a lot of args together as an array (vs as a hash, may make sense to just pass p1 etc. in a hash, would likely reduce overhead, but I have not investigated)
• example-usage is how you would compose with a component with slots in a template.

What do these look like before AST transforms?

<vbox>
  {{#if slots.header}}
    <box fit>
      {{slot 'header' header 'hello' 'world'}}
    </box>
  {{/if}}

  <hbox>
    {{slot 'main' body 'goodnight' 'moon'}}
  </hbox>

  {{#if slots.footer}}
    <box fit>
      {{slot 'footer' footer 'awesome' 'you'}}
    </box>
  {{/if}}
</vbox>

<box>
        {{#faked-yields as |info|}}
          I am the content {{info.title}}
        {{/faked-yields}}
      </box>
      <box>
        {{#faked-yields}}
          {{#slot "header" as |info|}}
            I am the content {{info.title}}
          {{/slot}}
          {{#slot "main" as |info|}}
            I am the content {{info.title}}
          {{/slot}}
          {{#slot "footer" as |info|}}
            I am the content {{info.title}}
          {{/slot}}
        {{/faked-yields}}