Extend reserved name check

Changelog.md

@@ -5,6 +5,7 @@ Enhancements:
 
 * Improve pluralisation of words ending with `s` (like process). (Joshua Pinter, #2779)
 * Add ordering by file modification time (most recent first). (Matheus Richard, #2778)
+* Extend reserved memoized helper name checking for #let and #subject. (Nick Flückiger, #2886)
 
 Bug fixes:
 

lib/rspec/core/memoized_helpers.rb

@@ -307,9 +307,13 @@ def let(name, &block)
           # We have to pass the block directly to `define_method` to
           # allow it to use method constructs like `super` and `return`.
           raise "#let or #subject called without a block" if block.nil?
-          raise(
-            "#let or #subject called with a reserved name #initialize"
-          ) if :initialize == name
+
+          # A list of reserved words that can't be used as a name for a memoized helper
+          # Matches for both symbols and passed strings
+          if [:initialize, :to_s].include?(name.to_sym)
+            raise ArgumentError, "#let or #subject called with reserved name `#{name}`"
+          end
+
           our_module = MemoizedHelpers.module_for(self)
 
           # If we have a module clash in our helper module

spec/rspec/core/memoized_helpers_spec.rb

@@ -520,10 +520,28 @@ def count
       end.to raise_error(/#let or #subject called without a block/)
     end
 
-    it 'raises an error when attempting to define a reserved method name' do
+    it 'raises an error when attempting to define a reserved name #initialize' do
       expect do
-        RSpec.describe { let(:initialize) { true }}
-      end.to raise_error(/#let or #subject called with a reserved name #initialize/)
+        RSpec.describe { let(:initialize) { true } }
+      end.to raise_error(/#let or #subject called with reserved name `initialize`/)
+    end
+
+    it 'raises an error when attempting to define a reserved name #initialize as a string' do
+      expect do
+        RSpec.describe { let('initialize') { true } }
+      end.to raise_error(/#let or #subject called with reserved name `initialize`/)
+    end
+
+    it 'raises an error when attempting to define a reserved name #to_s' do
+      expect do
+        RSpec.describe { let(:to_s) { true } }
+      end.to raise_error(/#let or #subject called with reserved name `to_s`/)
+    end
+
+    it 'raises an error when attempting to define a reserved name #to_s as a string' do
+      expect do
+        RSpec.describe { let('to_s') { true } }
+      end.to raise_error(/#let or #subject called with reserved name `to_s`/)
     end
 
     let(:a_value) { "a string" }