support resolvable partial intersecting peerSets

In cases where a peerSet _partially_ overlaps another, we were
previously treating that as a conflict, because not everything in the
peerSet was being replaced.

So for example,

```
root -> (a)
a -> PEER(w@1, x@1, y@1)
b -> PEER(x@1.0, y@1.0, z@1.0)
```

First we install, and get `w@1.1.0`, `x@1.1.0` and `z@1.1.0`.

Then, we try to install `b`, and need to install the `1.0.0` versions of
`x` and `y`.  We could replace `x` and `y`, but then `z` had no
replacement, and was treated as an ERESOLVE mistakenly.

Some work was required to make it still note when a not-replaced member
of the peer set _is_ indicative of a conflict, because it depends on a
version of a dep within the peer set that is not satisfied by the one
being replaced.

Fix: npm/cli#3152

PR-URL: #272
Credit: @isaacs
Close: #272
Reviewed-by: @nlf

lib/arborist/build-ideal-tree.js

@@ -1631,32 +1631,137 @@ This is a one-time fix-up, please be patient...
         // placed here as well.  the virtualRoot already has the appropriate
         // overrides applied.
         if (peerEntryEdge) {
-          const peerSet = getPeerSet(current)
-          OUTER: for (const p of peerSet) {
-            // if any have a non-peer dep from the target, or a peer dep if
-            // the target is root, then cannot safely replace and dupe deeper.
+          const currentPeerSet = getPeerSet(current)
+
+          // We are effectively replacing currentPeerSet with newPeerSet
+          // If there are any non-peer deps coming into the currentPeerSet,
+          // which are currently valid, and are from the target, then that
+          // means that we have to ensure that they're not going to be made
+          // invalid by putting the newPeerSet in place.
+          // If the edge comes from somewhere deeper than the target, then
+          // that's fine, because we'll create an invalid edge, detect it,
+          // and duplicate the node further into the tree.
+          // loop through the currentPeerSet checking for valid edges on
+          // the members of the peer set which will be made invalid.
+          const targetEdges = new Set()
+          for (const p of currentPeerSet) {
             for (const edge of p.edgesIn) {
-              if (peerSet.has(edge.from))
+              // edge from within the peerSet, ignore
+              if (currentPeerSet.has(edge.from))
                 continue
+              // only care about valid edges from target.
+              // edges from elsewhere can dupe if offended, invalid edges
+              // are already being fixed or will be later.
+              if (edge.from !== target || !edge.valid)
+                continue
+              targetEdges.add(edge)
+            }
+          }
 
-              // only respect valid edges, however, since we're likely trying
-              // to fix the very one that's currently broken!  If the virtual
-              // root's replacement is ok, and doesn't have any invalid edges
-              // indicating that it was an overridden peer, then ignore the
-              // conflict and continue.  If it WAS an override, then we need
-              // to get the conflict here so that we can decide whether to
-              // accept the current dep node, clobber it, or fail the install.
-              if (edge.from === target && edge.valid) {
-                const rep = dep.parent.children.get(edge.name)
-                const override = rep && ([...rep.edgesIn].some(e => !e.valid))
-                if (!rep || !rep.satisfies(edge) || override) {
+          for (const edge of targetEdges) {
+            // see if we intend to replace this one anyway
+            const rep = dep.parent.children.get(edge.name)
+            const current = edge.to
+            if (!rep) {
+              // this isn't one we're replacing.  but it WAS included in the
+              // peerSet for some reason, so make sure that it's still
+              // ok with the replacements in the new peerSet
+              for (const curEdge of current.edgesOut.values()) {
+                const newRepDep = dep.parent.children.get(curEdge.name)
+                if (curEdge.valid && newRepDep && !newRepDep.satisfies(curEdge)) {
                   canReplace = false
-                  break OUTER
+                  break
                 }
               }
+              continue
+            }
+
+            // was this replacement already an override of some sort?
+            const override = [...rep.edgesIn].some(e => !e.valid)
+            // if we have a rep, and it's ok to put in this location, and
+            // it's not already part of an override in the peerSet, then
+            // we can continue with it.
+            if (rep.satisfies(edge) && !override)
+              continue
+            // Otherwise, we cannot replace.
+            canReplace = false
+            break
+          }
+          // if we're going to be replacing the peerSet, we have to remove
+          // and re-resolve any members of the old peerSet that are not
+          // present in the new one, and which will have invalid edges.
+          // We know that they're not depended upon by the target, or else
+          // they would have caused a conflict, so they'll get landed deeper
+          // in the tree, if possible.
+          if (canReplace) {
+            let needNesting = false
+            OUTER: for (const node of currentPeerSet) {
+              const rep = dep.parent.children.get(node.name)
+              // has a replacement, already addressed above
+              if (rep)
+                continue
+
+              // ok, it has been placed here to dedupe, see if it needs to go
+              // back deeper within the tree.
+              for (const edge of node.edgesOut.values()) {
+                const repDep = dep.parent.children.get(edge.name)
+                // not in new peerSet, maybe fine.
+                if (!repDep)
+                  continue
+
+                // new thing will be fine, no worries
+                if (repDep.satisfies(edge))
+                  continue
+
+                // uhoh, we'll have to nest them.
+                needNesting = true
+                break OUTER
+              }
+            }
+
+            // to nest, just delete everything without a target dep
+            // that's in the current peerSet, and add their dependants
+            // to the _depsQueue for evaluation.  Some of these MAY end
+            // up in the same location again, and that's fine.
+            if (needNesting) {
+              // avoid mutating the tree while we're examining it
+              const dependants = new Set()
+              const reresolve = new Set()
+              OUTER: for (const node of currentPeerSet) {
+                const rep = dep.parent.children.get(node.name)
+                if (rep)
+                  continue
+                // create a separate set for each one, so we can skip any
+                // that might somehow have an incoming target edge
+                const deps = new Set()
+                for (const edge of node.edgesIn) {
+                  // a target dep, skip this dep entirely, already addressed
+                  // ignoring for coverage, because it really ought to be
+                  // impossible, but I can't prove it yet, so this is here
+                  // for safety.
+                  /* istanbul ignore if - should be impossible */
+                  if (edge.from === target)
+                    continue OUTER
+                  // ignore this edge, it'll either be replaced or re-resolved
+                  if (currentPeerSet.has(edge.from))
+                    continue
+                  // ok, we care about this one.
+                  deps.add(edge.from)
+                }
+                reresolve.add(node)
+                for (const d of deps)
+                  dependants.add(d)
+              }
+              for (const dependant of dependants) {
+                this[_depsQueue].push(dependant)
+                this[_depsSeen].delete(dependant)
+              }
+              for (const node of reresolve)
+                node.root = null
             }
           }
         }
+
         if (canReplace) {
           const ret = this[_canPlacePeers](dep, target, edge, REPLACE, peerEntryEdge, peerPath, isSource)
           /* istanbul ignore else - extremely rare that the peer set would