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diff --git a/spec/bundler/resolver/basic_spec.rb b/spec/bundler/resolver/basic_spec.rb
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+++ b/spec/bundler/resolver/basic_spec.rb
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+# frozen_string_literal: true
+require "spec_helper"
+
+RSpec.describe "Resolving" do
+  before :each do
+    @index = an_awesome_index
+  end
+
+  it "resolves a single gem" do
+    dep "rack"
+
+    should_resolve_as %w(rack-1.1)
+  end
+
+  it "resolves a gem with dependencies" do
+    dep "actionpack"
+
+    should_resolve_as %w(actionpack-2.3.5 activesupport-2.3.5 rack-1.0)
+  end
+
+  it "resolves a conflicting index" do
+    @index = a_conflict_index
+    dep "my_app"
+    should_resolve_as %w(activemodel-3.2.11 builder-3.0.4 grape-0.2.6 my_app-1.0.0)
+  end
+
+  it "resolves a complex conflicting index" do
+    @index = a_complex_conflict_index
+    dep "my_app"
+    should_resolve_as %w(a-1.4.0 b-0.3.5 c-3.2 d-0.9.8 my_app-1.1.0)
+  end
+
+  it "resolves a index with conflict on child" do
+    @index = index_with_conflict_on_child
+    dep "chef_app"
+    should_resolve_as %w(berkshelf-2.0.7 chef-10.26 chef_app-1.0.0 json-1.7.7)
+  end
+
+  it "resolves a index with root level conflict on child" do
+    @index = a_index_with_root_conflict_on_child
+    dep "i18n", "~> 0.4"
+    dep "activesupport", "~> 3.0"
+    dep "activerecord", "~> 3.0"
+    dep "builder", "~> 2.1.2"
+    should_resolve_as %w(activesupport-3.0.5 i18n-0.4.2 builder-2.1.2 activerecord-3.0.5 activemodel-3.0.5)
+  end
+
+  it "raises an exception if a child dependency is not resolved" do
+    @index = a_unresovable_child_index
+    dep "chef_app_error"
+    expect do
+      resolve
+    end.to raise_error(Bundler::VersionConflict)
+  end
+
+  it "raises an exception with the minimal set of conflicting dependencies" do
+    @index = build_index do
+      %w(0.9 1.0 2.0).each {|v| gem("a", v) }
+      gem("b", "1.0") { dep "a", ">= 2" }
+      gem("c", "1.0") { dep "a", "< 1" }
+    end
+    dep "a"
+    dep "b"
+    dep "c"
+    expect do
+      resolve
+    end.to raise_error(Bundler::VersionConflict, <<-E.strip)
+Bundler could not find compatible versions for gem "a":
+  In Gemfile:
+    b was resolved to 1.0, which depends on
+      a (>= 2)
+
+    c was resolved to 1.0, which depends on
+      a (< 1)
+    E
+  end
+
+  it "should throw error in case of circular dependencies" do
+    @index = a_circular_index
+    dep "circular_app"
+
+    expect do
+      resolve
+    end.to raise_error(Bundler::CyclicDependencyError, /please remove either gem 'bar' or gem 'foo'/i)
+  end
+
+  # Issue #3459
+  it "should install the latest possible version of a direct requirement with no constraints given" do
+    @index = a_complicated_index
+    dep "foo"
+    should_resolve_and_include %w(foo-3.0.5)
+  end
+
+  # Issue #3459
+  it "should install the latest possible version of a direct requirement with constraints given" do
+    @index = a_complicated_index
+    dep "foo", ">= 3.0.0"
+    should_resolve_and_include %w(foo-3.0.5)
+  end
+
+  it "takes into account required_ruby_version" do
+    @index = build_index do
+      gem "foo", "1.0.0" do
+        dep "bar", ">= 0"
+      end
+
+      gem "foo", "2.0.0" do |s|
+        dep "bar", ">= 0"
+        s.required_ruby_version = "~> 2.0.0"
+      end
+
+      gem "bar", "1.0.0"
+
+      gem "bar", "2.0.0" do |s|
+        s.required_ruby_version = "~> 2.0.0"
+      end
+
+      gem "ruby\0", "1.8.7"
+    end
+    dep "foo"
+    dep "ruby\0", "1.8.7"
+
+    deps = []
+    @deps.each do |d|
+      deps << Bundler::DepProxy.new(d, "ruby")
+    end
+
+    should_resolve_and_include %w(foo-1.0.0 bar-1.0.0), [{}, []]
+  end
+
+  context "conservative" do
+    before :each do
+      @index = build_index do
+        gem("foo", "1.3.7") { dep "bar", "~> 2.0" }
+        gem("foo", "1.3.8") { dep "bar", "~> 2.0" }
+        gem("foo", "1.4.3") { dep "bar", "~> 2.0" }
+        gem("foo", "1.4.4") { dep "bar", "~> 2.0" }
+        gem("foo", "1.4.5") { dep "bar", "~> 2.1" }
+        gem("foo", "1.5.0") { dep "bar", "~> 2.1" }
+        gem("foo", "1.5.1") { dep "bar", "~> 3.0" }
+        gem("foo", "2.0.0") { dep "bar", "~> 3.0" }
+        gem "bar", %w(2.0.3 2.0.4 2.0.5 2.1.0 2.1.1 3.0.0)
+      end
+      dep "foo"
+
+      # base represents declared dependencies in the Gemfile that are still satisfied by the lockfile
+      @base = Bundler::SpecSet.new([])
+
+      # locked represents versions in lockfile
+      @locked = locked(%w(foo 1.4.3), %w(bar 2.0.3))
+    end
+
+    it "resolves all gems to latest patch" do
+      # strict is not set, so bar goes up a minor version due to dependency from foo 1.4.5
+      should_conservative_resolve_and_include :patch, [], %w(foo-1.4.5 bar-2.1.1)
+    end
+
+    it "resolves all gems to latest patch strict" do
+      # strict is set, so foo can only go up to 1.4.4 to avoid bar going up a minor version, and bar can go up to 2.0.5
+      should_conservative_resolve_and_include [:patch, :strict], [], %w(foo-1.4.4 bar-2.0.5)
+    end
+
+    it "resolves foo only to latest patch - same dependency case" do
+      @locked = locked(%w(foo 1.3.7), %w(bar 2.0.3))
+      # bar is locked, and the lock holds here because the dependency on bar doesn't change on the matching foo version.
+      should_conservative_resolve_and_include :patch, ["foo"], %w(foo-1.3.8 bar-2.0.3)
+    end
+
+    it "resolves foo only to latest patch - changing dependency not declared case" do
+      # foo is the only gem being requested for update, therefore bar is locked, but bar is NOT
+      # declared as a dependency in the Gemfile. In this case, locks don't apply to _changing_
+      # dependencies and since the dependency of the selected foo gem changes, the latest matching
+      # dependency of "bar", "~> 2.1" -- bar-2.1.1 -- is selected. This is not a bug and follows
+      # the long-standing documented Conservative Updating behavior of bundle install.
+      # http://bundler.io/v1.12/man/bundle-install.1.html#CONSERVATIVE-UPDATING
+      should_conservative_resolve_and_include :patch, ["foo"], %w(foo-1.4.5 bar-2.1.1)
+    end
+
+    it "resolves foo only to latest patch - changing dependency declared case" do
+      # bar is locked AND a declared dependency in the Gemfile, so it will not move, and therefore
+      # foo can only move up to 1.4.4.
+      @base << build_spec("bar", "2.0.3").first
+      should_conservative_resolve_and_include :patch, ["foo"], %w(foo-1.4.4 bar-2.0.3)
+    end
+
+    it "resolves foo only to latest patch strict" do
+      # adding strict helps solve the possibly unexpected behavior of bar changing in the prior test case,
+      # because no versions will be returned for bar ~> 2.1, so the engine falls back to ~> 2.0 (turn on
+      # debugging to see this happen).
+      should_conservative_resolve_and_include [:patch, :strict], ["foo"], %w(foo-1.4.4 bar-2.0.3)
+    end
+
+    it "resolves bar only to latest patch" do
+      # bar is locked, so foo can only go up to 1.4.4
+      should_conservative_resolve_and_include :patch, ["bar"], %w(foo-1.4.3 bar-2.0.5)
+    end
+
+    it "resolves all gems to latest minor" do
+      # strict is not set, so bar goes up a major version due to dependency from foo 1.4.5
+      should_conservative_resolve_and_include :minor, [], %w(foo-1.5.1 bar-3.0.0)
+    end
+
+    it "resolves all gems to latest minor strict" do
+      # strict is set, so foo can only go up to 1.5.0 to avoid bar going up a major version
+      should_conservative_resolve_and_include [:minor, :strict], [], %w(foo-1.5.0 bar-2.1.1)
+    end
+
+    it "resolves all gems to latest major" do
+      should_conservative_resolve_and_include :major, [], %w(foo-2.0.0 bar-3.0.0)
+    end
+
+    it "resolves all gems to latest major strict" do
+      should_conservative_resolve_and_include [:major, :strict], [], %w(foo-2.0.0 bar-3.0.0)
+    end
+
+    # Why would this happen in real life? If bar 2.2 has a bug that the author of foo wants to bypass
+    # by reverting the dependency, the author of foo could release a new gem with an older requirement.
+    context "revert to previous" do
+      before :each do
+        @index = build_index do
+          gem("foo", "1.4.3") { dep "bar", "~> 2.2" }
+          gem("foo", "1.4.4") { dep "bar", "~> 2.1.0" }
+          gem("foo", "1.5.0") { dep "bar", "~> 2.0.0" }
+          gem "bar", %w(2.0.5 2.1.1 2.2.3)
+        end
+        dep "foo"
+
+        # base represents declared dependencies in the Gemfile that are still satisfied by the lockfile
+        @base = Bundler::SpecSet.new([])
+
+        # locked represents versions in lockfile
+        @locked = locked(%w(foo 1.4.3), %w(bar 2.2.3))
+      end
+
+      it "could revert to a previous version level patch" do
+        should_conservative_resolve_and_include :patch, [], %w(foo-1.4.4 bar-2.1.1)
+      end
+
+      it "cannot revert to a previous version in strict mode level patch" do
+        # the strict option removes the version required to match, so a version conflict results
+        expect do
+          should_conservative_resolve_and_include [:patch, :strict], [], %w(foo-1.4.3 bar-2.1.1)
+        end.to raise_error Bundler::VersionConflict, /#{Regexp.escape("Could not find gem 'bar (~> 2.1.0)'")}/
+      end
+
+      it "could revert to a previous version level minor" do
+        should_conservative_resolve_and_include :minor, [], %w(foo-1.5.0 bar-2.0.5)
+      end
+
+      it "cannot revert to a previous version in strict mode level minor" do
+        # the strict option removes the version required to match, so a version conflict results
+        expect do
+          should_conservative_resolve_and_include [:minor, :strict], [], %w(foo-1.4.3 bar-2.1.1)
+        end.to raise_error Bundler::VersionConflict, /#{Regexp.escape("Could not find gem 'bar (~> 2.0.0)'")}/
+      end
+    end
+  end
+end