Improve handling of constants that are just aliases

 * In the constant summary, link to its target object
 * When linking from other locations, link directly to the
   target.

lib/yard/code_objects/constant_object.rb

@@ -12,5 +12,23 @@ class ConstantObject < Base
     def value=(value)
       @value = format_source(value)
     end
+
+    # @return [Base, nil] the target object the constant points to
+    def target
+      return @target if instance_variable_defined?(:@target)
+
+      if !value.empty? &&
+        (target = P(namespace, value)) &&
+        !target.is_a?(YARD::CodeObjects::Proxy) &&
+        target != self
+        @target = target
+      else
+        @target = nil
+      end
+      @target
+    rescue YARD::Parser::UndocumentableError
+      # means the value isn't an alias to another object
+      @target = nil
+    end
   end
 end

lib/yard/templates/helpers/html_helper.rb

@@ -301,6 +301,17 @@ def insert_include(text, markup = options.markup)
       def link_object(obj, title = nil, anchor = nil, relative = true)
         return title if obj.nil?
         obj = Registry.resolve(object, obj, true, true) if obj.is_a?(String)
+
+        was_const = false
+        # Re-link references to constants that are aliases to their target. But keep
+        # their current title.
+        while obj.is_a?(CodeObjects::ConstantObject) && obj.target
+          title ||= h(object.relative_path(obj)).to_s
+          was_const = true
+          obj = obj.target
+        end
+        return link_object(obj, title, anchor, relative) if was_const
+
         if title
           title = title.to_s
         elsif object.is_a?(CodeObjects::Base)

spec/code_objects/constant_object_spec.rb

@@ -0,0 +1,48 @@
+# frozen_string_literal: true
+require File.dirname(__FILE__) + '/spec_helper'
+
+RSpec.describe YARD::CodeObjects::ConstantObject do
+  before do
+    Registry.clear
+  end
+
+  describe "#target" do
+    it "resolves" do
+        const1 = ConstantObject.new(:root, :A)
+        const2 = ConstantObject.new(:root, :B)
+        const2.value = "A"
+        expect(const2.target).to eq const1
+    end
+
+    it "returns nil for an integer value" do
+        const = ConstantObject.new(:root, :A)
+        const.value = "1"
+        expect(const.target).to be_nil
+    end
+
+    it "returns nil for a string value" do
+        const = ConstantObject.new(:root, :A)
+        const.value = '"String"'
+        expect(const.target).to be_nil
+    end
+
+    it "returns nil for an empty value" do
+        const = ConstantObject.new(:root, :A)
+        const.value = ""
+        expect(const.target).to be_nil
+    end
+
+    it "returns nil for an explicit self-referential constant" do
+        const = ConstantObject.new(:root, :A)
+        const.value = "A"
+        expect(const.target).to be_nil
+    end
+
+    it "returns nil for an explicit self-referential constant" do
+        mod = ModuleObject.new(:root, :M)
+        const = ConstantObject.new(mod, :A)
+        const.value = "self"
+        expect(const.target).to be_nil
+    end
+  end
+end

spec/templates/examples/module006.html

@@ -0,0 +1,54 @@
+<h1>Module: A
+
+
+
+</h1>
+<div class="box_info">
+
+
+
+
+
+
+
+
+
+
+
+  <dl>
+    <dt>Defined in:</dt>
+    <dd>(stdin)</dd>
+  </dl>
+
+</div>
+
+<h2>Overview</h2><div class="docstring">
+  <div class="discussion">
+    Checkout BAR
+
+  </div>
+</div>
+<div class="tags">
+
+
+</div>
+
+    <h2>
+      Constant Summary
+      <small><a href="#" class="constants_summary_toggle">collapse</a></small>
+    </h2>
+
+    <dl class="constants">
+
+        <dt id="FOO-constant" class="">FOO =
+
+        </dt>
+        <dd><pre class="code">1</pre></dd>
+
+        <dt id="BAR-constant" class="">BAR =
+
+        </dt>
+        <dd>FOO</dd>
+
+    </dl>
+

spec/templates/module_spec.rb

@@ -200,4 +200,16 @@ module A
     eof
     html_equals(Registry.at('A').format(html_options), :module005)
   end
+
+  it "renders constants that are aliases as links in html" do
+    Registry.clear
+    YARD.parse_string <<~RUBY
+      # Checkout {BAR}
+      module A
+        FOO = 1
+        BAR = FOO
+      end
+    RUBY
+    html_equals(Registry.at('A').format(html_options), :module006)
+  end
 end

templates/default/module/html/constant_summary.erb

@@ -10,7 +10,7 @@
         <dt id="<%= anchor_for(cnst) %>" class="<%= cnst.has_tag?(:deprecated) ? 'deprecated' : '' %>"><%= cnst.name %> =
           <%= yieldall :object => cnst %>
         </dt>
-        <dd><pre class="code"><%= format_constant cnst.value %></pre></dd>
+        <dd><% if cnst.is_a?(YARD::CodeObjects::ConstantObject) && cnst.target %><%= linkify(cnst.target, cnst.value) %><% else %><pre class="code"><%= format_constant cnst.value %></pre><% end %></dd>
       <% end %>
     </dl>
   <% end %>