Prioritize path.system over path when it makes sense

If `path.system` is configured locally, and `path` is configured
globally, gems should be installed to `path.system`.

bundler/lib/bundler/settings.rb

@@ -177,6 +177,16 @@ def locations(key)
       locations
     end
 
+    def first_location_for(key)
+      key = key_for(key)
+
+      return :temporary if @temporary.key?(key)
+      return :local if @local_config.key?(key)
+      return :env if ENV[key]
+      return :global if @global_config.key?(key)
+      return :default if DEFAULT_CONFIG.key?(key)
+    end
+
     def pretty_values_for(exposed_key)
       key = key_for(exposed_key)
 
@@ -204,12 +214,6 @@ def pretty_values_for(exposed_key)
 
     # for legacy reasons, in Bundler 2, we do not respect :disable_shared_gems
     def path
-      key  = key_for(:path)
-      path = ENV[key] || @global_config[key]
-      if path && !@temporary.key?(key) && !@local_config.key?(key)
-        return Path.new(path, false, false)
-      end
-
       system_path = self["path.system"] || (self[:disable_shared_gems] == false)
       Path.new(self[:path], system_path, Bundler.feature_flag.default_install_uses_path?)
     end
@@ -249,7 +253,11 @@ def base_path_relative_to_pwd
       end
 
       def validate!
-        return unless explicit_path && system_path
+        first_path_location = Bundler.settings.first_location_for(:path)
+        return if first_path_location.nil?
+        first_system_path_location = Bundler.settings.first_location_for("path.system")
+        return if first_system_path_location.nil?
+        return unless first_path_location == first_system_path_location
         path = Bundler.settings.pretty_values_for(:path)
         path.unshift(nil, "path:") unless path.empty?
         system_path = Bundler.settings.pretty_values_for("path.system")

bundler/spec/install/path_spec.rb

@@ -19,6 +19,14 @@
       expect(the_bundle).to include_gems "rack 1.0.0"
     end
 
+    it "uses system gems with `path.system` configured with more priority than `path`" do
+      bundle "config --local path.system true"
+      bundle "config --global path vendor/bundle"
+      bundle :install
+      run "require 'rack'", :raise_on_error => false
+      expect(out).to include("FAIL")
+    end
+
     it "handles paths with regex characters in them" do
       dir = bundled_app("bun++dle")
       dir.mkpath