Uncalled Function Checks

New to 3.7 is a check inside if statements for when you accidentally use a function instead of the return value of a function. This only applies when the function is known to exist making the if statement always be true. Here is an example plugin interface, where there are optional and non-optional callbacks. 

interface PluginSettings {
  pluginShouldLoad?: () => void;
  pluginIsActivated: () => void;
}

declare const plugin: PluginSettings;
// Because pluginShouldLoad could not exist, then the check is legitimate. 
if (plugin.pluginShouldLoad) {
  // Do something when pluginShouldLoad exists.
}

// In 3.6 and below, this was not an error.

if (plugin.pluginIsActivated) {
  // Want to do something when the plugin is activated,
  // but instead of calling the method we used it as a
  // property.
}
// pluginIsActivated should always exist, but TypeScript still allows the check, because the method is called inside the if block. 
if (plugin.pluginIsActivated) {
  plugin.pluginIsActivated();
}