Remove AliasType#types? and guardrails for cyclic hierarchies

src/compiler/crystal/codegen/link.cr

@@ -292,8 +292,6 @@ module Crystal
 
     private def add_link_annotations(types, annotations)
       types.try &.each_value do |type|
-        next if type.is_a?(AliasType) || type.is_a?(TypeDefType)
-
         if type.is_a?(LibType) && type.used? && (link_annotations = type.link_annotations)
           annotations.concat link_annotations
         end

src/compiler/crystal/semantic/abstract_def_checker.cr

@@ -41,7 +41,10 @@ class Crystal::AbstractDefChecker
   end
 
   def check_single(type)
-    return if @all_checked.includes?(type)
+    if @all_checked.includes?(type)
+      ::raise "AbstractDefChecker duplicate: #{type} (#{type.class})"
+      return
+    end
     @all_checked << type
 
     if type.abstract? || type.module?

src/compiler/crystal/semantic/new.cr

@@ -22,8 +22,6 @@ module Crystal
     end
 
     def define_default_new(type)
-      return if type.is_a?(AliasType) || type.is_a?(TypeDefType)
-
       type.types?.try &.each_value do |type|
         define_default_new_single(type)
       end

src/compiler/crystal/semantic/recursive_struct_checker.cr

@@ -35,7 +35,10 @@ class Crystal::RecursiveStructChecker
 
   def check_single(type)
     has_not_been_checked = @all_checked.add?(type)
-    return unless has_not_been_checked
+    unless has_not_been_checked
+      raise "RecursiveStructChecker duplicate: #{type} (#{type.class})"
+      return
+    end
 
     if struct?(type)
       target = type

src/compiler/crystal/semantic/type_declaration_processor.cr

@@ -627,7 +627,10 @@ struct Crystal::TypeDeclarationProcessor
   end
 
   private def remove_duplicate_instance_vars_declarations(type : Type, duplicates_checked : Set(Type))
-    return unless duplicates_checked.add?(type)
+    unless duplicates_checked.add?(type)
+      raise "remove_duplicate_instance_vars_declarations duplicate: #{type} (#{type.class})"
+      return
+    end
 
     # If a class has an instance variable that already exists in a superclass, remove it.
     # Ideally we should process instance variables in a top-down fashion, but it's tricky

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

@@ -251,13 +251,6 @@ 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

src/compiler/crystal/tools/typed_def_processor.cr

@@ -17,15 +17,6 @@ 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

@@ -128,15 +128,6 @@ 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

src/compiler/crystal/types.cr

@@ -2760,19 +2760,6 @@ module Crystal
     delegate lookup_defs, lookup_defs_with_modules, lookup_first_def,
       lookup_macro, lookup_macros, to: aliased_type
 
-    def types?
-      process_value
-      if aliased_type = @aliased_type
-        aliased_type.types?
-      else
-        nil
-      end
-    end
-
-    def types
-      types?.not_nil!
-    end
-
     def lookup_name(name)
       process_value
       @aliased_type.try(&.lookup_name(name))