4 files changed, 34 insertions, 2 deletions
diff --git a/actionmailer/CHANGELOG.md b/actionmailer/CHANGELOG.md
index 2836f0cfbc..04bbae8495 100644
--- a/actionmailer/CHANGELOG.md
+++ b/actionmailer/CHANGELOG.md
@@ -1,3 +1,9 @@
+*   Bring back proc with arity of 1 in `ActionMailer::Base.default` proc
+    since it was supported in Rails 5.0 but not deprecated.
+
+    *Jimmy Bourassa*
+
+
 ## Rails 5.2.0.beta2 (November 28, 2017) ##
 
 *   No changes.
diff --git a/actionmailer/lib/action_mailer/base.rb b/actionmailer/lib/action_mailer/base.rb
index eb8ae59533..3af95081ee 100644
--- a/actionmailer/lib/action_mailer/base.rb
+++ b/actionmailer/lib/action_mailer/base.rb
@@ -889,7 +889,7 @@ module ActionMailer
         default_values = self.class.default.map do |key, value|
           [
             key,
-            value.is_a?(Proc) ? instance_exec(&value) : value
+            compute_default(value)
           ]
         end.to_h
 
@@ -898,6 +898,16 @@ module ActionMailer
         headers_with_defaults
       end
 
+      def compute_default(value)
+        return value unless value.is_a?(Proc)
+
+        if value.arity == 1
+          instance_exec(self, &value)
+        else
+          instance_exec(&value)
+        end
+      end
+
       def assign_headers_to_message(message, headers)
         assignable = headers.except(:parts_order, :content_type, :body, :template_name,
                                     :template_path, :delivery_method, :delivery_method_options)
diff --git a/actionmailer/test/base_test.rb b/actionmailer/test/base_test.rb
index 977e0e201e..ac90809518 100644
--- a/actionmailer/test/base_test.rb
+++ b/actionmailer/test/base_test.rb
@@ -725,6 +725,15 @@ 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