Fix AM::Base.default proc arity breaking change

PR #29270 changed the number of arguments that gets passed to Procs
defined in ActionMail::Base.default. With this changeset, Procs can
now have 1 or 0 arguments

Also adds test coverage for AM::Base.default Proc arity.

2 files changed, 17 insertions, 1 deletions

diff --git a/actionmailer/test/base_test.rb b/actionmailer/test/base_test.rb
index 0da969c349..6f3b638994 100644
--- a/actionmailer/test/base_test.rb
+++ b/actionmailer/test/base_test.rb
@@ -726,6 +726,15 @@ class BaseTest < ActiveSupport::TestCase
     assert(ProcMailer.welcome["x-has-to-proc"].to_s == "symbol")
   end
 
+  test "proc default values can have arity of 1 where arg is a mailer instance" do
+    assert_equal(ProcMailer.welcome['X-Lambda-Arity-1-arg'].to_s, "complex_value")
+    assert_equal(ProcMailer.welcome['X-Lambda-Arity-1-self'].to_s, "complex_value")
+  end
+
+  test "proc default values with fixed arity of 0 can be called" do
+    assert_equal("0", ProcMailer.welcome["X-Lambda-Arity-0"].to_s)
+  end
+
   test "we can call other defined methods on the class as needed" do
     mail = ProcMailer.welcome
     assert_equal("Thanks for signing up this afternoon", mail.subject)
diff --git a/actionmailer/test/mailers/proc_mailer.rb b/actionmailer/test/mailers/proc_mailer.rb
index b7cf53eb4a..76e730bb79 100644
--- a/actionmailer/test/mailers/proc_mailer.rb
+++ b/actionmailer/test/mailers/proc_mailer.rb
@@ -4,12 +4,19 @@ class ProcMailer < ActionMailer::Base
   default to: "system@test.lindsaar.net",
           "X-Proc-Method" => Proc.new { Time.now.to_i.to_s },
           subject: Proc.new { give_a_greeting },
-          "x-has-to-proc" => :symbol
+          "x-has-to-proc" => :symbol,
+          "X-Lambda-Arity-0" => ->() { "0" },
+          "X-Lambda-Arity-1-arg" => ->(arg) { arg.computed_value },
+          "X-Lambda-Arity-1-self" => ->(_) { self.computed_value }
 
   def welcome
     mail
   end
 
+  def computed_value
+    "complex_value"
+  end
+
   private
 
     def give_a_greeting