[ruby/irb] Improve constant lookup in SourceFinder

(https://github.com/ruby/irb/pull/871)

https://github.com/ruby/irb/commit/87c279ccf2

lib/irb/source_finder.rb

@@ -69,10 +69,18 @@ module IRB
 
     def find_source(signature, super_level = 0)
       case signature
-      when /\A(::)?[A-Z]\w*(::[A-Z]\w*)*\z/ # Const::Name
+      when /\A(::)?[A-Z]\w*(::[A-Z]\w*)*\z/ # ConstName, ::ConstName, ConstPath::ConstName
         eval_receiver_or_owner(signature) # trigger autoload
-        base = @irb_context.workspace.binding.receiver.yield_self { |r| r.is_a?(Module) ? r : Object }
-        file, line = base.const_source_location(signature)
+        *parts, name = signature.split('::', -1)
+        base =
+          if parts.empty? # ConstName
+            find_const_owner(name)
+          elsif parts == [''] # ::ConstName
+            Object
+          else # ConstPath::ConstName
+            eval_receiver_or_owner(parts.join('::'))
+          end
+        file, line = base.const_source_location(name)
       when /\A(?<owner>[A-Z]\w*(::[A-Z]\w*)*)#(?<method>[^ :.]+)\z/ # Class#method
         owner = eval_receiver_or_owner(Regexp.last_match[:owner])
         method = Regexp.last_match[:method]
@@ -122,5 +130,10 @@ module IRB
     rescue NameError
       raise EvaluationError
     end
+
+    def find_const_owner(name)
+      module_nesting = @irb_context.workspace.binding.eval('::Module.nesting')
+      module_nesting.find { |mod| mod.const_defined?(name, false) } || module_nesting.find { |mod| mod.const_defined?(name) } || Object
+    end
   end
 end

test/irb/cmd/test_show_source.rb

@@ -362,5 +362,36 @@ module TestIRB
       refute_match(/NameError/, out)
       assert_match(%r[Defined in binary file:.+io/console], out)
     end
+
+    def test_show_source_with_constant_lookup
+      write_ruby <<~RUBY
+        X = 1
+        module M
+          Y = 1
+          Z = 2
+        end
+        class A
+          Z = 1
+          Array = 1
+          class B
+            include M
+            Object.new.instance_eval { binding.irb }
+          end
+        end
+      RUBY
+
+      out = run_ruby_file do
+        type "show_source X"
+        type "show_source Y"
+        type "show_source Z"
+        type "show_source Array"
+        type "exit"
+      end
+
+      assert_match(%r[#{@ruby_file.to_path}:1\s+X = 1], out)
+      assert_match(%r[#{@ruby_file.to_path}:3\s+Y = 1], out)
+      assert_match(%r[#{@ruby_file.to_path}:7\s+Z = 1], out)
+      assert_match(%r[#{@ruby_file.to_path}:8\s+Array = 1], out)
+    end
   end
 end