Exploration in Discord: https://discord.com/channels/480462759797063690/568935504288940056/1432791167799656469

The problem is here:

• ember-cli/lib/models/command.js

  Line 315 in 01a68b9

  if (this.hasOption('watcher')) {

  
  We never enter this block unless a watcher option is present. when doing ember s, watcher has a default value of "events", specified here:

  ember-cli/lib/commands/serve.js

  Line 57 in 01a68b9

  { name: 'watcher', type: String, default: 'events', aliases: ['w'] },

  -- but in the build command (edited in this PR) -- we were omitting that in the availableOptions during $EMBROIDER_PREBUILD -- which opts the build --watch that compatPrebuild is doing out of using watchman (which is a requirement to do anything on macOS with watched files).

NOTE: availableOptions was found as being relevant while debugging parseArgs

• ember-cli/lib/models/command.js

  Line 554 in 01a68b9

  return this.assignOption(option, parsedOptions, commandOptions);

  -- where you find if you break in that function, you never see watcher -- which is required to use watchman.
  • from here:

    ember-cli/lib/models/command.js

    Line 584 in 01a68b9

    if (!this.availableOptions.every(assembleAndValidateOption.bind(this))) {

An alternate implementation of this PR is to always use watchman if process.env.EMBROIDER_PREBUILD is present (or rather, assume we "have a watcher" -- and let our call to WatchDetector figure it out -- (Linux, for example, doesn't need watchman))