feat(arborist)!: install optional peer dependencies

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

@@ -1200,23 +1200,21 @@ This is a one-time fix-up, please be patient...
           return false
         }
 
+        // If the edge has no destination, that's a problem
+        if (!edge.to) {
+          return true
+        }
+
         // If it's already been logged as a load failure, skip it.
-        if (edge.to && this[_loadFailures].has(edge.to)) {
+        if (this[_loadFailures].has(edge.to)) {
           return false
         }
 
         // If it's shrinkwrapped, we use what the shrinkwap wants.
-        if (edge.to && edge.to.inShrinkwrap) {
+        if (edge.to.inShrinkwrap) {
           return false
         }
 
-        // If the edge has no destination, that's a problem, unless
-        // if it's peerOptional and not explicitly requested.
-        if (!edge.to) {
-          return edge.type !== 'peerOptional' ||
-            this[_explicitRequests].has(edge)
-        }
-
         // If the edge has an error, there's a problem.
         if (!edge.valid) {
           return true

workspaces/arborist/lib/place-dep.js

@@ -496,7 +496,27 @@ class PlaceDep {
   }
 
   get conflictOk () {
-    return this.force || (!this.isMine && !this.strictPeerDeps)
+    if (this.force) {
+      return true
+    }
+
+    if (!this.isMine && !this.strictPeerDeps) {
+      return true
+    }
+
+    if (this.top.edge.type === 'peerOptional') {
+      return true
+    }
+
+    let fromIsOptional = !!this.top.edge.from.edgesIn.size
+    for (const edgeIn of this.top.edge.from.edgesIn.values()) {
+      if (edgeIn.type !== 'peerOptional') {
+        fromIsOptional = false
+        break
+      }
+    }
+
+    return fromIsOptional
   }
 
   get isMine () {