From 66542956562f54aa70fc48be89965e9d2d4770a9 Mon Sep 17 00:00:00 2001
From: Yusuke Endoh <mame@ruby-lang.org>
Date: Fri, 3 Apr 2026 19:32:29 +0900
Subject: [PATCH] Support .nil? narrowing for local and instance variables

`x.nil?` in conditions now narrows the variable type: the then
branch treats x as nil, and the else branch excludes nil. Works
with both local variables and instance variables.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
---
 lib/typeprof/core/ast/call.rb | 14 ++++++++++++++
 scenario/flow/nil_check.rb    | 13 +++++++++++++
 2 files changed, 27 insertions(+)
 create mode 100644 scenario/flow/nil_check.rb

diff --git a/lib/typeprof/core/ast/call.rb b/lib/typeprof/core/ast/call.rb
index d228e823..96685150 100644
--- a/lib/typeprof/core/ast/call.rb
+++ b/lib/typeprof/core/ast/call.rb
@@ -270,6 +270,20 @@ def narrowings
             else
               super
             end
+          when :nil?
+            if @recv.is_a?(LocalVariableReadNode)
+              [
+                Narrowing.new({ @recv.var => Narrowing::NilConstraint.new(true) }),
+                Narrowing.new({ @recv.var => Narrowing::NilConstraint.new(false) })
+              ]
+            elsif @recv.is_a?(InstanceVariableReadNode)
+              [
+                Narrowing.new({ @recv.var => Narrowing::NilConstraint.new(true) }),
+                Narrowing.new({ @recv.var => Narrowing::NilConstraint.new(false) })
+              ]
+            else
+              super
+            end
           when :!
             then_narrowing, else_narrowing = @recv.narrowings
             [else_narrowing, then_narrowing]
diff --git a/scenario/flow/nil_check.rb b/scenario/flow/nil_check.rb
new file mode 100644
index 00000000..6920817d
--- /dev/null
+++ b/scenario/flow/nil_check.rb
@@ -0,0 +1,13 @@
+## update
+def foo(x)
+  unless x.nil?
+    x.to_sym
+  end
+end
+foo(nil)
+foo("hello")
+
+## assert
+class Object
+  def foo: (String?) -> Symbol?
+end