Support implicit pub dependencies

lib/src/solver/incompatibility.dart

@@ -109,6 +109,26 @@ class Incompatibility {
 
       return "${_terse(depender, details, allowEvery: true)} depends on "
           "${_terse(dependee, details)}";
+    } else if (cause == IncompatibilityCause.pubDependency) {
+      if (terms.length == 1) {
+        var forbidden = terms.first;
+        assert(forbidden.isPositive);
+
+        // The only one-term pub dependency is on barback, which forbids
+        // versions outside the range pub supports.
+        return "pub itself depends on ${_terseRef(forbidden, details)} "
+            "${VersionConstraint.any.difference(forbidden.constraint)}";
+      }
+
+      assert(terms.length == 2);
+      assert(terms.first.isPositive);
+      assert(terms.first.package.name == "barback");
+
+      var dependee = terms.last;
+      assert(!dependee.isPositive);
+
+      return "when barback is in use pub itself depends on " +
+          _terse(dependee, details);
     } else if (cause == IncompatibilityCause.useLatest) {
       assert(terms.length == 1);
 
@@ -222,16 +242,19 @@ class Incompatibility {
   /// that should be associated with [this] and [other], respectively.
   String andToString(Incompatibility other,
       [Map<String, PackageDetail> details, int thisLine, int otherLine]) {
-    var requiresBoth = _tryRequiresBoth(other, details, thisLine, otherLine);
-    if (requiresBoth != null) return requiresBoth;
-
-    var requiresThrough =
-        _tryRequiresThrough(other, details, thisLine, otherLine);
-    if (requiresThrough != null) return requiresThrough;
-
-    var requiresForbidden =
-        _tryRequiresForbidden(other, details, thisLine, otherLine);
-    if (requiresForbidden != null) return requiresForbidden;
+    if (this.cause != IncompatibilityCause.pubDependency &&
+        other.cause != IncompatibilityCause.pubDependency) {
+      var requiresBoth = _tryRequiresBoth(other, details, thisLine, otherLine);
+      if (requiresBoth != null) return requiresBoth;
+
+      var requiresThrough =
+          _tryRequiresThrough(other, details, thisLine, otherLine);
+      if (requiresThrough != null) return requiresThrough;
+
+      var requiresForbidden =
+          _tryRequiresForbidden(other, details, thisLine, otherLine);
+      if (requiresForbidden != null) return requiresForbidden;
+    }
 
     var buffer = new StringBuffer(this.toString(details));
     if (thisLine != null) buffer.write(" $thisLine");

lib/src/solver/incompatibility_cause.dart

@@ -19,6 +19,11 @@ abstract class IncompatibilityCause {
   /// The incompatibility represents a package's dependency.
   static const IncompatibilityCause dependency = const _Cause("dependency");
 
+  /// The incompatibility represents pub's own dependency, which is activated
+  /// when barback is selected.
+  static const IncompatibilityCause pubDependency =
+      const _Cause("pub dependency");
+
   /// The incompatibility represents the user's request that we use the latest
   /// version of a given package.
   static const IncompatibilityCause useLatest = const _Cause("use latest");

lib/src/solver/version_solver.dart

@@ -8,6 +8,7 @@ import 'dart:math' as math;
 import 'package:collection/collection.dart';
 import 'package:pub_semver/pub_semver.dart';
 
+import '../barback.dart';
 import '../exceptions.dart';
 import '../lock_file.dart';
 import '../log.dart' as log;
@@ -79,7 +80,29 @@ class VersionSolver {
   VersionSolver(this._type, this._systemCache, this._root, this._lockFile,
       Iterable<String> useLatest)
       : _overriddenPackages = new MapKeySet(_root.pubspec.dependencyOverrides),
-        _useLatest = new Set.from(useLatest);
+        _useLatest = new Set.from(useLatest) {
+    _addImplicitIncompatibilities();
+  }
+
+  /// Adds incompatibilities representing the dependencies pub itself has on
+  /// various packages to support barback at runtime.
+  void _addImplicitIncompatibilities() {
+    var barbackOverride = _root.pubspec.dependencyOverrides["barback"];
+    var barbackRef = barbackOverride == null
+        ? _systemCache.sources.hosted.refFor("barback")
+        : barbackOverride.toRef();
+
+    pubConstraints.forEach((name, constraint) {
+      if (_root.pubspec.dependencyOverrides.containsKey(name)) return;
+
+      _addIncompatibility(new Incompatibility([
+        new Term(barbackRef.withConstraint(VersionConstraint.any), true),
+        new Term(
+            _systemCache.sources.hosted.refFor(name).withConstraint(constraint),
+            false)
+      ], IncompatibilityCause.pubDependency));
+    });
+  }
 
   /// Finds a set of dependencies that match the root package's constraints, or
   /// throws an error if no such set is available.

test/implicit_barback_dependency_test.dart

@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-@Skip()
-
 import 'package:test/test.dart';
 
 import 'package:pub/src/barback.dart' as barback;
@@ -150,10 +148,11 @@ main() {
 
     await d.appDir({"barback": "any"}).create();
 
-    await pubGet(error: """
-Package barback has no versions that match >=$current <$max derived from:
-- myapp depends on version any
-- pub itself depends on version >=$current <$max""");
+    await pubGet(error: equalsIgnoringWhitespace("""
+      Because no versions of barback match >=0.15.0 <0.15.3 and pub itself
+        depends on barback >=0.15.0 <0.15.3, barback is forbidden.
+      So, because myapp depends on barback any, version solving failed.
+    """));
   });
 
   test(
@@ -169,9 +168,9 @@ Package barback has no versions that match >=$current <$max derived from:
 
     await d.appDir({"barback": previous}).create();
 
-    await pubGet(error: """
-Incompatible version constraints on barback:
-- myapp depends on version $previous
-- pub itself depends on version >=$current <$max""");
+    await pubGet(error: equalsIgnoringWhitespace("""
+      Because pub itself depends on barback >=0.15.0 <0.15.3 and myapp depends
+        on barback 0.14.0, version solving failed.
+    """));
   });
 }

test/implicit_dependency_test.dart

@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-@Skip()
-
 import 'package:test/test.dart';
 
 import 'package:pub/src/barback.dart' as barback;