feat: Instantiate exception classes and set missing backtrace in reject.

config/flog.yml

@@ -1,2 +1,2 @@
 ---
-threshold: 9.7
+threshold: 10.6

config/reek.yml

@@ -60,7 +60,7 @@ TooManyInstanceVariables:
 TooManyMethods:
   enabled: true
   exclude: []
-  max_methods: 12
+  max_methods: 13
 TooManyStatements:
   enabled: true
   exclude:

lib/promise.rb

@@ -63,7 +63,7 @@ def fulfill(value = nil)
   def reject(reason = nil)
     dispatch do
       @state = :rejected
-      @reason = reason || Error.new.tap { |err| err.set_backtrace(caller) }
+      @reason = reason_coercion(reason || Error)
     end
   end
 
@@ -73,6 +73,16 @@ def defer
 
   private
 
+  def reason_coercion(reason)
+    case reason
+    when Exception
+      reason.set_backtrace(caller) unless reason.backtrace
+    when Class
+      reason = reason_coercion(reason.new) if reason <= Exception
+    end
+    reason
+  end
+
   def add_callback(&generator)
     if pending?
       @callbacks << generator

spec/unit/promise_spec.rb

@@ -8,8 +8,9 @@
   let(:value) { double('value') }
   let(:other_value) { double('other_value') }
 
+  let(:backtrace) { caller }
   let(:reason) do
-    StandardError.new('reason').tap { |err| err.set_backtrace(caller) }
+    StandardError.new('reason').tap { |err| err.set_backtrace(backtrace) }
   end
   let(:other_reason) do
     StandardError.new('other_reason').tap { |err| err.set_backtrace(caller) }
@@ -400,6 +401,26 @@
         expect(subject.reason.backtrace.join)
           .to include(__FILE__ + ':' + (__LINE__ - 2).to_s)
       end
+
+      it 'leaves backtrace if already set' do
+        subject.reject(reason)
+        expect(subject.reason.backtrace).to eq(backtrace)
+      end
+
+      it 'instantiates exception class' do
+        subject.reject(Exception)
+        expect(subject.reason).to be_a(Exception)
+      end
+
+      it 'instantiates exception subclasses' do
+        subject.reject(RuntimeError)
+        expect(subject.reason).to be_a(RuntimeError)
+      end
+
+      it "doesn't instantiate non-error classes" do
+        subject.reject(Hash)
+        expect(subject.reason).to eq(Hash)
+      end
     end
 
     describe '#sync' do