[UI::Shell] Warn on STDERR in Bundler 2

2 files changed, 17 insertions, 3 deletions

diff --git a/lib/bundler/ui/shell.rb b/lib/bundler/ui/shell.rb
index a1a2cbad18..e0d0d68aa9 100644
--- a/lib/bundler/ui/shell.rb
+++ b/lib/bundler/ui/shell.rb
@@ -30,9 +30,12 @@ module Bundler
       end
 
       def warn(msg, newline = nil)
+        return unless level("warn")
         return if @warning_history.include? msg
         @warning_history << msg
-        tell_me(msg, :yellow, newline) if level("warn")
+
+        return tell_err(msg, :yellow, newline) if Bundler.feature_flag.error_on_stderr?
+        tell_me(msg, :yellow, newline)
       end
 
       def error(msg, newline = nil)
diff --git a/spec/bundler/ui/shell_spec.rb b/spec/bundler/ui/shell_spec.rb
index 6585ab1da8..9a47a3572f 100644
--- a/spec/bundler/ui/shell_spec.rb
+++ b/spec/bundler/ui/shell_spec.rb
@@ -21,9 +21,20 @@ RSpec.describe Bundler::UI::Shell do
 
   describe "#warn" do
     before { subject.level = "warn" }
-    it "prints to stdout" do
+    it "prints to stdout", :bundler => "< 2" do
       expect { subject.warn("warning") }.to output("warning\n").to_stdout
     end
+
+    it "prints to stderr", :bundler => "2" do
+      expect { subject.warn("warning") }.to output("warning\n").to_stderr
+    end
+
+    context "when stderr flag is enabled" do
+      before { Bundler.settings.temporary(:error_on_stderr => true) }
+      it "prints to stderr" do
+        expect { subject.warn("warning!") }.to output("warning!\n").to_stderr
+      end
+    end
   end
 
   describe "#debug" do
@@ -35,7 +46,7 @@ RSpec.describe Bundler::UI::Shell do
   describe "#error" do
     before { subject.level = "error" }
 
-    it "prints to stdout", :bundler => "<= 2" do
+    it "prints to stdout", :bundler => "< 2" do
       expect { subject.error("error!!!") }.to output("error!!!\n").to_stdout
     end