fix: prune optional peer dependencies that are no longer explicitly depended on

workspaces/arborist/lib/arborist/build-ideal-tree.js

@@ -1465,19 +1465,19 @@ This is a one-time fix-up, please be patient...
     const needPrune = metaFromDisk && (mutateTree || flagsSuspect)
     if (this.#prune && needPrune) {
       this.#idealTreePrune()
-      for (const node of this.idealTree.inventory.values()) {
-        if (node.extraneous) {
-          node.parent = null
-        }
-      }
     }
 
     timeEnd()
   }
 
   #idealTreePrune () {
     for (const node of this.idealTree.inventory.values()) {
-      if (node.extraneous) {
+      // optional peer dependencies are meant to be added to the tree
+      // through an explicit required dependency (most commonly in the
+      // root package.json), at which point they won't be optional so
+      // any dependencies still marked as both optional and peer at
+      // this point can be pruned as a special kind of extraneous
+      if (node.extraneous || (node.peer && node.optional)) {
         node.parent = null
       }
     }

workspaces/arborist/test/arborist/pruner.js

@@ -1,4 +1,5 @@
 const { resolve } = require('node:path')
+const fs = require('node:fs')
 
 const t = require('tap')
 const Arborist = require('../../lib/arborist/index.js')
@@ -39,6 +40,23 @@ t.test('prune with lockfile', async t => {
   t.matchSnapshot(printTree(tree))
 })
 
+t.test('prune with lockfile with implicit optional peer dependencies', async t => {
+  const path = fixture(t, 'prune-lockfile-optional-peer')
+  const tree = await pruneTree(path, { audit: false })
+
+  const dep = tree.children.get('dedent')
+  t.ok(dep, 'required prod dep was not pruned from tree')
+
+  const optionalPeerDep = tree.children.get('babel-plugin-macros')
+  t.notOk(optionalPeerDep, 'optional peer dep was pruned from tree')
+
+  t.notMatch(
+    fs.readFileSync(path + '/package-lock.json'),
+    'node_modules/babel-plugin-macros',
+    'should remove optional peer dep from package-lock.json'
+  )
+})
+
 t.test('prune with actual tree omit dev', async t => {
   const path = fixture(t, 'prune-actual-omit-dev')
   const tree = await pruneTree(path, { omit: ['dev'] })

workspaces/arborist/test/fixtures/prune-lockfile-optional-peer/package.json

@@ -0,0 +1,7 @@
+{
+  "name": "prune-lockfile-optional-peer",
+  "version": "1.0.0",
+  "dependencies": {
+    "dedent": "^1.6.0"
+  }
+}

workspaces/arborist/test/fixtures/reify-cases/prune-lockfile-optional-peer.js

@@ -0,0 +1,60 @@
+// generated from test/fixtures/prune-lockfile-optional-peer
+module.exports = t => {
+  const path = t.testdir({
+  "node_modules": {
+    "dedent": {
+      "package.json": JSON.stringify({
+        "name": "dedent",
+        "version": "1.6.0",
+        "peerDependencies": {
+          "babel-plugin-macros": "^3.1.0"
+        },
+        "peerDependenciesMeta": {
+          "babel-plugin-macros": {
+            "optional": true
+          }
+        }
+      })
+    }
+  },
+  "package-lock.json": JSON.stringify({
+    "name": "prune-lockfile-optional-peer",
+    "version": "1.0.0",
+    "lockfileVersion": 3,
+    "requires": true,
+    "packages": {
+      "": {
+        "name": "prune-lockfile-optional-peer",
+        "version": "1.0.0",
+        "dependencies": {
+          "dedent": "^1.6.0"
+        }
+      },
+      "node_modules/babel-plugin-macros": {
+        "version": "3.1.0",
+        "optional": true,
+        "peer": true
+      },
+      "node_modules/dedent": {
+        "version": "1.6.0",
+        "peerDependencies": {
+          "babel-plugin-macros": "^3.1.0"
+        },
+        "peerDependenciesMeta": {
+          "babel-plugin-macros": {
+            "optional": true
+          }
+        }
+      }
+    }
+  }),
+  "package.json": JSON.stringify({
+    "name": "prune-lockfile-optional-peer",
+    "version": "1.0.0",
+    "dependencies": {
+      "dedent": "^1.6.0"
+    }
+  })
+  })
+  return path
+}