Fix: unbound transitive dependencies should not conflict with top lev…

…el dependency (yarnpkg#4488)

**Summary**

Fixes yarnpkg#3780, and makes the failing test from yarnpkg#3779 passing.

As a final step of package resolution, for each dependency we check whether any version satisfies all resolved version ranges. 

**Test plan**

Fixes an existing (failing) test: "unbound transitive dependencies should not conflict with top level dependency"

__tests__/commands/install/integration.js

@@ -1049,8 +1049,7 @@ test.concurrent('transitive file: dependencies should work', (): Promise<void> =
   });
 });
 
-// Unskip once https://github.com/yarnpkg/yarn/issues/3778 is resolved
-test.skip('unbound transitive dependencies should not conflict with top level dependency', async () => {
+test('unbound transitive dependencies should not conflict with top level dependency', async () => {
   await runInstall({flat: true}, 'install-conflicts', async config => {
     expect((await fs.readJson(path.join(config.cwd, 'node_modules', 'left-pad', 'package.json'))).version).toEqual(
       '1.0.0',

src/package-resolver.js

@@ -228,6 +228,14 @@ export default class PackageResolver {
 
   getAllInfoForPackageName(name: string): Array<Manifest> {
     const patterns = this.patternsByPackage[name] || [];
+    return this.getAllInfoForPatterns(patterns);
+  }
+
+  /**
+   * Retrieve all the package info stored for a list of patterns.
+   */
+
+  getAllInfoForPatterns(patterns: string[]): Array<Manifest> {
     const infos = [];
     const seen = new Set();
 
@@ -284,6 +292,13 @@ export default class PackageResolver {
 
   collapseAllVersionsOfPackage(name: string, version: string): string {
     const patterns = this.dedupePatterns(this.patternsByPackage[name]);
+    return this.collapsePackageVersions(name, version, patterns);
+  }
+
+  /**
+   * Make all given patterns resolve to version.
+   */
+  collapsePackageVersions(name: string, version: string, patterns: string[]): string {
     const human = `${name}@${version}`;
 
     // get manifest that matches the version we're collapsing too
@@ -530,10 +545,37 @@ export default class PackageResolver {
       this.resolveToResolution(req);
     }
 
+    for (const dep of deps) {
+      const name = normalizePattern(dep.pattern).name;
+      this.optimizeResolutions(name);
+    }
+
     activity.end();
     this.activity = null;
   }
 
+  // for a given package, see if a single manifest can satisfy all ranges
+  optimizeResolutions(name: string) {
+    const patterns: Array<string> = this.dedupePatterns(this.patternsByPackage[name] || []);
+
+    // don't optimize things that already have a lockfile entry:
+    // https://github.com/yarnpkg/yarn/issues/79
+    const collapsablePatterns = patterns.filter(pattern => {
+      const remote = this.patterns[pattern]._remote;
+      return !this.lockfile.getLocked(pattern) && (!remote || remote.type !== 'workspace');
+    });
+    if (collapsablePatterns.length < 2) {
+      return;
+    }
+
+    const availableVersions = this.getAllInfoForPatterns(collapsablePatterns).map(manifest => manifest.version);
+    const combinedRange = collapsablePatterns.map(pattern => normalizePattern(pattern).range).join(' ');
+    const singleVersion = semver.maxSatisfying(availableVersions, combinedRange);
+    if (singleVersion) {
+      this.collapsePackageVersions(name, singleVersion, collapsablePatterns);
+    }
+  }
+
   /**
     * Called by the package requester for packages that this resolver already had
     * a matching version for. Delay the resolve, because better matches can still be