From bca3ea0756c4a224afc1a65809b59ed3faec0d3d Mon Sep 17 00:00:00 2001
From: Takumi Shotoku <sinsoku.listy@gmail.com>
Date: Thu, 20 Nov 2025 21:49:30 +0900
Subject: [PATCH] Fix type inference for Class.new assigned to constants
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When a constant is assigned the result of Class.new (e.g.,
MyError = Class.new(StandardError)), TypeProf now correctly infers
the constant as a singleton type instead of a Class instance.

This fixes a NoMethodError when using such dynamically created
classes in rescue clauses with assignment (rescue MyError => e).

Changes:
- Detect Class.new pattern in ConstantWriteNode#install0
- Create singleton type for the constant when Class.new is detected
- Support both Class.new and ::Class.new patterns

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>
---
 lib/typeprof/core/ast/const.rb                | 22 ++++++++++++++++++-
 .../control/rescue-assign-with-class-new.rb   | 15 +++++++++++++
 2 files changed, 36 insertions(+), 1 deletion(-)
 create mode 100644 scenario/control/rescue-assign-with-class-new.rb

diff --git a/lib/typeprof/core/ast/const.rb b/lib/typeprof/core/ast/const.rb
index 927f05cb..2943a68f 100644
--- a/lib/typeprof/core/ast/const.rb
+++ b/lib/typeprof/core/ast/const.rb
@@ -112,10 +112,30 @@ def install0(genv)
         @cpath.install(genv) if @cpath
         val = @rhs.install(genv)
         if @static_cpath
-          @changes.add_edge(genv, val, @static_ret.vtx)
+          if detect_class_new?(@rhs)
+            # Create a module entity for the dynamically created class
+            mod = genv.resolve_cpath(@static_cpath)
+
+            # Create singleton type for the new class
+            singleton_ty = Type::Singleton.new(genv, mod)
+            vtx = Source.new(singleton_ty)
+            @changes.add_edge(genv, vtx, @static_ret.vtx)
+          else
+            @changes.add_edge(genv, val, @static_ret.vtx)
+          end
         end
         val
       end
+
+      private
+
+      def detect_class_new?(node)
+        node.is_a?(CallBaseNode) &&
+          node.mid == :new &&
+          node.recv.is_a?(ConstantReadNode) &&
+          node.recv.cname == :Class &&
+          node.recv.cbase.nil?
+      end
     end
   end
 end
diff --git a/scenario/control/rescue-assign-with-class-new.rb b/scenario/control/rescue-assign-with-class-new.rb
new file mode 100644
index 00000000..ebfc82c7
--- /dev/null
+++ b/scenario/control/rescue-assign-with-class-new.rb
@@ -0,0 +1,15 @@
+## update: my_error.rb
+MyError = Class.new(StandardError)
+
+## update: test.rb
+class C
+  def foo
+  rescue MyError => e
+    raise ArgumentError, e.message
+  end
+end
+
+## assert: test.rb
+class C
+  def foo: -> nil
+end