From 86851655acffd9bf15c41d09feea2bf266b7cccb Mon Sep 17 00:00:00 2001
From: Takumi Shotoku <sinsoku.listy@gmail.com>
Date: Sat, 5 Sep 2026 17:47:30 +0900
Subject: [PATCH] Give the RBS instance type its type arguments

Type.default_param_map built the `*instance` entry with an empty argument
list, so a singleton method declared to return `instance` lost its type
arguments, and crashed when that result reached a type variable check
(SigTyVarNode#typecheck got nil instead of an actual argument). After
base_type the receiver is always an Instance or a Singleton, so
get_instance_type is always defined.

    table = Hash[[["a", "b"]]]  # Hash.[] is declared as `-> instance`
    "foo".gsub(/o/, table)      # gsub takes `hash[String, _ToS]`
---
 lib/typeprof/core/type.rb     |  2 +-
 scenario/rbs/instance-type.rb | 25 +++++++++++++++++++++++++
 2 files changed, 26 insertions(+), 1 deletion(-)
 create mode 100644 scenario/rbs/instance-type.rb

diff --git a/lib/typeprof/core/type.rb b/lib/typeprof/core/type.rb
index 5450608aa..7ddf86346 100644
--- a/lib/typeprof/core/type.rb
+++ b/lib/typeprof/core/type.rb
@@ -48,7 +48,7 @@ def self.collect_hash_value_types(type)
 
     def self.default_param_map(genv, ty)
       ty = ty.base_type(genv)
-      instance_ty = ty.is_a?(Type::Instance) ? ty : Type::Instance.new(genv, ty.mod, []) # TODO: type params
+      instance_ty = ty.is_a?(Type::Instance) ? ty : ty.get_instance_type(genv)
       singleton_ty = ty.is_a?(Type::Instance) ? Type::Singleton.new(genv, ty.mod) : ty
       {
         "*self": Source.new(ty),
diff --git a/scenario/rbs/instance-type.rb b/scenario/rbs/instance-type.rb
new file mode 100644
index 000000000..aa8cc504c
--- /dev/null
+++ b/scenario/rbs/instance-type.rb
@@ -0,0 +1,25 @@
+## update: test.rbs
+class Gen[T]
+  def self.create: () -> instance
+end
+
+type gen[T] = Gen[T]
+
+class Object
+  def accept: (gen[Integer]) -> String
+end
+
+## update: test.rb
+def test
+  accept(Gen.create)
+end
+
+def raw
+  Gen.create
+end
+
+## assert
+class Object
+  def test: -> String
+  def raw: -> Gen[untyped]
+end