install: Filter optional deps during diff-trees

This allows `--no-optional` runs to include optional dependencies in the
lockfile while excluding them from installation on disk.

lib/install/deps.js

@@ -489,12 +489,6 @@ function loadDeps (tree, log, next) {
   if (!tree.package.dependencies) tree.package.dependencies = {}
   asyncMap(Object.keys(tree.package.dependencies), function (dep, done) {
     var version = tree.package.dependencies[dep]
-    if (tree.package.optionalDependencies &&
-        tree.package.optionalDependencies[dep] &&
-        !npm.config.get('optional')) {
-      return done()
-    }
-
     addDependency(dep, version, tree, log.newGroup('loadDep:' + dep), andHandleOptionalErrors(log, tree, dep, done))
   }, andForEachChild(loadDeps, andFinishTracker(log, next)))
 }

lib/install/diff-trees.js

@@ -8,6 +8,7 @@ var log = require('npmlog')
 var path = require('path')
 var ssri = require('ssri')
 var moduleName = require('../utils/module-name.js')
+var isOnlyOptional = require('./is-only-optional.js')
 
 // we don't use get-requested because we're operating on files on disk, and
 // we don't want to extropolate from what _should_ be there.
@@ -234,18 +235,26 @@ var diffTrees = module.exports._diffTrees = function (oldTree, newTree) {
     .map((flatname) => toRemove[flatname])
     .forEach((pkg) => setAction(differences, 'remove', pkg))
 
+  return filterActions(differences)
+}
+
+function filterActions (differences) {
+  const includeOpt = npm.config.get('optional')
   const includeDev = npm.config.get('dev') ||
     (!/^prod(uction)?$/.test(npm.config.get('only')) && !npm.config.get('production')) ||
     /^dev(elopment)?$/.test(npm.config.get('only')) ||
     /^dev(elopment)?$/.test(npm.config.get('also'))
   const includeProd = !/^dev(elopment)?$/.test(npm.config.get('only'))
-  if (!includeProd || !includeDev) {
-    log.silly('diff-trees', 'filtering actions:', 'includeDev', includeDev, 'includeProd', includeProd)
-    differences = differences.filter((diff) => {
-      const pkg = diff[1]
-      const pkgIsOnlyDev = isOnlyDev(pkg)
-      return (!includeProd && pkgIsOnlyDev) || (includeDev && pkgIsOnlyDev) || (includeProd && !pkgIsOnlyDev)
-    })
-  }
-  return differences
+  if (includeProd && includeDev && includeOpt) return differences
+
+  log.silly('diff-trees', 'filtering actions:', 'includeDev', includeDev, 'includeProd', includeProd, 'includeOpt', includeOpt)
+  return differences.filter((diff) => {
+    const pkg = diff[1]
+    const pkgIsOnlyDev = isOnlyDev(pkg)
+    const pkgIsOnlyOpt = isOnlyOptional(pkg)
+    if (!includeProd && pkgIsOnlyDev) return true
+    if (includeDev && pkgIsOnlyDev) return true
+    if (includeProd && !pkgIsOnlyDev && (includeOpt || !pkgIsOnlyOpt)) return true
+    return false
+  })
 }