Docs: don't show subtypes of aliased type

spec/compiler/crystal/tools/doc/type_spec.cr

@@ -0,0 +1,31 @@
+require "../../../spec_helper"
+
+describe Doc::Type do
+  it "doesn't show types for alias type" do
+    result = semantic(%(
+      class Foo
+        class Bar
+        end
+      end
+
+      alias Alias = Foo
+
+      Alias
+    ))
+
+    program = result.program
+
+    # Set locations to types relative to the included dir
+    # so they are included by the doc generator
+    foo_bar_type = program.types["Foo"].types["Bar"]
+    foo_bar_type.add_location(Location.new("./foo.cr", 1, 1))
+
+    alias_type = program.types["Alias"]
+    alias_type.add_location(Location.new("./foo.cr", 1, 1))
+
+    generator = Doc::Generator.new program, ["."], ".", nil
+
+    doc_alias_type = generator.type(alias_type)
+    doc_alias_type.types.size.should eq(0)
+  end
+end

src/compiler/crystal/tools/doc/generator.cr

@@ -223,6 +223,13 @@ class Crystal::Doc::Generator
   def collect_subtypes(parent)
     types = [] of Type
 
+    # AliasType has defined `types?` to be the types
+    # of the aliased type, but for docs we don't want
+    # to list the nested types for aliases.
+    if parent.is_a?(AliasType)
+      return types
+    end
+
     parent.types?.try &.each_value do |type|
       case type
       when Const, LibType