New Twiddle

components.foo-bar.js

import Ember from 'ember';

export default Ember.Component.extend({
  tagName: ''
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle'
});

helpers.foo.js

import Ember from 'ember';

export function foo() {
  return "foo helper";
}

export default Ember.Helper.helper(foo);

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>

{{#let (component "foo-bar") as |foo|}}
  {{concat (foo bar)}} (should be runtime error?)
{{/let}}

<hr />

{{#let (component "foo-bar") as |foo|}}
  {{concat (foo)}} (should be runtime error?)
{{/let}}

<hr />

{{#let (component "foo-bar") as |foo|}}
  {{concat foo}} (should be [object Object])
{{/let}}

<hr />

{{#let "foo-bar string" as |foo|}}
  {{concat (foo bar)}} (should be runtime error?)
{{/let}}

<hr />

{{#let "foo-bar string" as |foo|}}
  {{concat (foo)}} (should be runtime error?)
{{/let}}

<hr />

{{#let "foo-bar string" as |foo|}}
  {{concat foo}} (should be foo-bar string)
{{/let}}
<br>
<br>

templates.components.foo-bar.hbs

foo-bar component

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}