diff --git a/lib/typeprof/core/ast/meta.rb b/lib/typeprof/core/ast/meta.rb
index a219a184f..f3c2ff69b 100644
--- a/lib/typeprof/core/ast/meta.rb
+++ b/lib/typeprof/core/ast/meta.rb
@@ -305,8 +305,9 @@ def subnodes = { block_body: }
       def attrs = { static_cpath:, members:, kind: }
 
       # Interface expected by MethodDefBox
-      def req_positionals = @kind == :struct ? @members : []
-      def opt_positionals = []
+      def req_positionals = []
+      # Struct.new(:x).new is valid, so every member is optional
+      def opt_positionals = @kind == :struct ? @members : []
       def rest_positionals = nil
       def post_positionals = []
       def req_keywords = @kind == :data ? @members : []
@@ -323,6 +324,10 @@ def define0(genv)
           ive = genv.resolve_ivar(struct_base_cpath, false, member)
           ive.add_def(self)
         end
+        if @kind == :struct
+          # Struct.[] calls the receiver's initialize, like Class#new
+          genv.resolve_method(struct_base_cpath, true, :[]).builtin = Builtin.new(genv).method(:class_new)
+        end
         @block_body.define(genv) if @block_body
         cdef
       end
@@ -345,11 +350,16 @@ def define_copy(genv)
       def undefine0(genv)
         mod = genv.resolve_cpath(@static_cpath)
         mod.remove_module_def(genv, self)
-        genv.resolve_cpath(struct_base_cpath).remove_module_def(genv, self)
+        base = genv.resolve_cpath(struct_base_cpath)
+        base.remove_module_def(genv, self)
         @members.each do |member|
           ive = genv.resolve_ivar(struct_base_cpath, false, member)
           ive.remove_def(self)
         end
+        # On an update, the new node has already set the builtin
+        if base.module_defs.to_a.none? { _1.kind == :struct }
+          genv.resolve_method(struct_base_cpath, true, :[]).builtin = nil
+        end
         @block_body.undefine(genv) if @block_body
       end
 
@@ -388,20 +398,13 @@ def install0(genv)
         end
         init_ret = @changes.add_escape_box(genv, Source.new(genv.nil_type))
         if @kind == :struct
-          init_f_args = FormalArguments.new(init_vtxs, [], nil, [], [], [], nil, nil)
+          init_f_args = FormalArguments.new([], init_vtxs, nil, [], [], [], nil, nil)
         else
           # Data.define uses keyword arguments
           init_f_args = FormalArguments.new([], [], nil, [], init_vtxs, [], nil, nil)
         end
         @changes.add_method_def_box(genv, cpath, false, :initialize, init_f_args, [init_ret])
 
-        # Struct.[] is an alias for Struct.new
-        if @kind == :struct
-          # Struct.[] builds the struct class itself, not the base class
-          self_ret = @changes.add_escape_box(genv, Source.new(Type::Instance.new(genv, genv.resolve_cpath(@static_cpath), [])))
-          @changes.add_method_def_box(genv, cpath, true, :[], init_f_args, [self_ret])
-        end
-
         # Install block body (additional method definitions)
         if @block_body
           @block_body.lenv.locals[:"*self"] = @block_body.lenv.cref.get_self(genv)
diff --git a/lib/typeprof/core/graph/box.rb b/lib/typeprof/core/graph/box.rb
index 68053326c..25cce3d97 100644
--- a/lib/typeprof/core/graph/box.rb
+++ b/lib/typeprof/core/graph/box.rb
@@ -840,7 +840,7 @@ def call(changes, genv, a_args, ret)
       end
     end
 
-    def show(output_parameter_names)
+    def show(output_parameter_names, ret: nil)
       block_show = []
       if @record_block.used
         blk_f_args = @record_block.f_args.map {|arg| arg.show }.join(", ")
@@ -891,7 +891,7 @@ def show(output_parameter_names)
       args = args.join(", ")
       s = args.empty? ? [] : ["(#{ args })"]
       s << "#{ block_show.sort.join(" | ") }" unless block_show.empty?
-      s << "-> #{ @mid == :initialize ? "void" : @ret.show }"
+      s << "-> #{ ret || (@mid == :initialize ? "void" : @ret.show) }"
       s.join(" ")
     end
   end
diff --git a/lib/typeprof/core/service.rb b/lib/typeprof/core/service.rb
index ed8077f9d..a71b44fea 100644
--- a/lib/typeprof/core/service.rb
+++ b/lib/typeprof/core/service.rb
@@ -520,6 +520,14 @@ def dump_declarations(path)
                         !@genv.resolve_method(node.static_cpath, mdef.singleton, mdef.mid).defs.empty?
                 out << "  " * stack.size + "def #{ mdef.singleton ? "self." : "" }#{ mdef.mid }: " + mdef.show(@options[:output_parameter_names])
               end
+              if node.is_a?(AST::StructNewNode) && node.kind == :struct &&
+                 @genv.resolve_method(node.static_cpath, true, :[]).defs.empty?
+                # Struct.[] takes the same arguments as the initialize it reaches
+                init_mdef = @genv.resolve_method(node.static_cpath, false, :initialize).defs.to_a.first ||
+                            @genv.resolve_method(node.struct_base_cpath, false, :initialize).defs.to_a.first
+                ret = Type::Instance.new(@genv, mod, []).show
+                out << "  " * stack.size + "def self.[]: " + init_mdef.show(@options[:output_parameter_names], ret:)
+              end
             else
               stack.pop
               out << "  " * stack.size + "end"
diff --git a/scenario/diagnostics/struct-aref.rb b/scenario/diagnostics/struct-aref.rb
new file mode 100644
index 000000000..c9e1aba81
--- /dev/null
+++ b/scenario/diagnostics/struct-aref.rb
@@ -0,0 +1,13 @@
+## update
+Pt = Struct.new(:x, :y) do
+  def initialize(x = 0, y = 0)
+    super
+  end
+end
+
+Pt[]
+Pt[3, 4]
+Pt[1, 2, 3]
+
+## diagnostics
+(9,2)-(9,11): wrong number of arguments (3 for 0...2)
diff --git a/scenario/diagnostics/struct-optional-members.rb b/scenario/diagnostics/struct-optional-members.rb
new file mode 100644
index 000000000..edab19be3
--- /dev/null
+++ b/scenario/diagnostics/struct-optional-members.rb
@@ -0,0 +1,10 @@
+## update
+Pt = Struct.new(:x, :y)
+
+Pt.new
+Pt.new(1)
+Pt[]
+Pt[1, 2, 3]
+
+## diagnostics
+(6,2)-(6,11): wrong number of arguments (3 for 0...2)
diff --git a/scenario/incremental/struct-new-block.rb b/scenario/incremental/struct-new-block.rb
index 4f5e606ee..f34057d30 100644
--- a/scenario/incremental/struct-new-block.rb
+++ b/scenario/incremental/struct-new-block.rb
@@ -8,8 +8,8 @@ def name: -> String
   def name=: (untyped) -> untyped
   def age: -> Integer
   def age=: (untyped) -> untyped
-  def initialize: (String, Integer) -> void
-  def self.[]: (String, Integer) -> Dog
+  def initialize: (?String, ?Integer) -> void
+  def self.[]: (?String, ?Integer) -> Dog
 end
 
 ## update
@@ -26,7 +26,7 @@ def name: -> String
   def name=: (untyped) -> untyped
   def age: -> Integer
   def age=: (untyped) -> untyped
-  def self.[]: (String, Integer) -> Dog
+  def self.[]: (String, String) -> Dog
   def initialize: (String, String) -> void
 end
 
@@ -40,6 +40,6 @@ def name: -> String
   def name=: (untyped) -> untyped
   def age: -> Integer
   def age=: (untyped) -> untyped
-  def initialize: (String, Integer) -> void
-  def self.[]: (String, Integer) -> Dog
+  def initialize: (?String, ?Integer) -> void
+  def self.[]: (?String, ?Integer) -> Dog
 end
diff --git a/scenario/misc/struct_new.rb b/scenario/misc/struct_new.rb
index af3f587c1..21df5dff6 100644
--- a/scenario/misc/struct_new.rb
+++ b/scenario/misc/struct_new.rb
@@ -12,8 +12,8 @@ def bar: -> Integer
   def bar=: (Integer) -> Integer
   def baz: -> String
   def baz=: (untyped) -> untyped
-  def initialize: (Integer, String) -> void
-  def self.[]: (Integer, String) -> Foo
+  def initialize: (?Integer, ?String) -> void
+  def self.[]: (?Integer, ?String) -> Foo
 end
 
 ## update
@@ -41,8 +41,8 @@ def double
 class Bar
   def n: -> Integer
   def n=: (untyped) -> untyped
-  def initialize: (Integer) -> void
-  def self.[]: (Integer) -> Bar
+  def initialize: (?Integer) -> void
+  def self.[]: (?Integer) -> Bar
   def double: -> Integer
 end
 
@@ -64,8 +64,8 @@ def ivar
 class Baz
   def v: -> Integer
   def v=: (untyped) -> untyped
-  def initialize: (Integer) -> void
-  def self.[]: (Integer) -> Baz
+  def initialize: (?Integer) -> void
+  def self.[]: (?Integer) -> Baz
   def set_label: -> String
   def ivar: -> String
 end
@@ -88,7 +88,7 @@ def x: -> Integer
   def x=: (untyped) -> untyped
   def y: -> Integer
   def y=: (untyped) -> untyped
-  def self.[]: (Integer, Integer) -> Pt
+  def self.[]: (?Integer, ?Integer) -> Pt
   def initialize: (?Integer, ?Integer) -> void
 end
 
@@ -106,7 +106,7 @@ def name: -> String
   def name=: (untyped) -> untyped
   def age: -> Integer
   def age=: (untyped) -> untyped
-  def self.[]: (String, Integer) -> Dog
+  def self.[]: (String, String) -> Dog
   def initialize: (String, String) -> void
 end
 
@@ -140,7 +140,7 @@ def x: -> Integer
   def x=: (untyped) -> untyped
   def y: -> Integer
   def y=: (untyped) -> untyped
-  def self.[]: (Integer, Integer) -> Pt
+  def self.[]: (?Integer, ?Integer) -> Pt
 end
 class Pt
   def initialize: (?Integer, ?Integer) -> void