Fix crystal tool unreachable & co visiting circular hierarchies (#1…

…5065)

Avoid visiting circular hierarchies created by type aliases.

spec/compiler/crystal/tools/unreachable_spec.cr

@@ -112,6 +112,14 @@ describe "unreachable" do
       CRYSTAL
   end
 
+  it "handles circular hierarchy references (#14034)" do
+    assert_unreachable <<-CRYSTAL
+      class Foo
+        alias Bar = Foo
+      end
+      CRYSTAL
+  end
+
   it "finds initializer" do
     assert_unreachable <<-CRYSTAL
       class Foo

src/compiler/crystal/tools/typed_def_processor.cr

@@ -17,6 +17,15 @@ module Crystal::TypedDefProcessor
   end
 
   private def process_type(type : Type) : Nil
+    # Avoid visiting circular hierarchies. There's no use in processing
+    # alias types anyway.
+    # For example:
+    #
+    #     struct Foo
+    #        alias Bar = Foo
+    #     end
+    return if type.is_a?(AliasType) || type.is_a?(TypeDefType)
+
     if type.is_a?(NamedType) || type.is_a?(Program) || type.is_a?(FileModule)
       type.types?.try &.each_value do |inner_type|
         process_type inner_type

src/compiler/crystal/tools/unreachable.cr

@@ -128,6 +128,15 @@ module Crystal
     property excludes = [] of String
 
     def process_type(type)
+      # Avoid visiting circular hierarchies. There's no use in processing
+      # alias types anyway.
+      # For example:
+      #
+      #     struct Foo
+      #        alias Bar = Foo
+      #     end
+      return if type.is_a?(AliasType) || type.is_a?(TypeDefType)
+
       if type.is_a?(ModuleType)
         track_unused_defs type
       end