From 4a4386cc415a997ad2d30da0a78d464fd898d983 Mon Sep 17 00:00:00 2001
From: ksss <co000ri@gmail.com>
Date: Mon, 18 Mar 2024 18:26:00 +0900
Subject: [PATCH 1/3] There is a possibility that nil is returned.

```rb
'a'.gsub!(/b/).each { }
```
---
 core/string.rbs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/core/string.rbs b/core/string.rbs
index 52dfa02ef..8ddab025b 100644
--- a/core/string.rbs
+++ b/core/string.rbs
@@ -2098,7 +2098,7 @@ class String
   # Related: String#sub, String#gsub, String#sub!.
   #
   def gsub!: (Regexp | string pattern, string | hash[String, _ToS] replacement) -> self?
-           | (Regexp | string pattern) -> Enumerator[String, self]
+           | (Regexp | string pattern) -> Enumerator[String, self?]
            | (Regexp | string pattern) { (String match) -> _ToS } -> self?
 
   # <!--

From e363ee5ebb614dd9371f746757d6371b97351ea4 Mon Sep 17 00:00:00 2001
From: ksss <co000ri@gmail.com>
Date: Mon, 18 Mar 2024 18:47:54 +0900
Subject: [PATCH 2/3] Support empty Enumerator

---
 lib/rbs/test/type_check.rb       | 17 ++++++++++-------
 test/rbs/test/type_check_test.rb |  2 ++
 test/stdlib/String_test.rb       |  2 --
 3 files changed, 12 insertions(+), 9 deletions(-)

diff --git a/lib/rbs/test/type_check.rb b/lib/rbs/test/type_check.rb
index cc9b89d56..872ea3d30 100644
--- a/lib/rbs/test/type_check.rb
+++ b/lib/rbs/test/type_check.rb
@@ -283,19 +283,22 @@ def value(val, type)
                 end
               end
 
-              each_sample(values).all? do |v|
+              value_check = values.empty? || each_sample(values).all? do |v|
                 if v.size == 1
                   # Only one block argument.
                   value(v[0], type.args[0]) || value(v, type.args[0])
                 else
                   value(v, type.args[0])
                 end
-              end &&
-                if ret.equal?(self)
-                  type.args[1].is_a?(Types::Bases::Bottom)
-                else
-                  value(ret, type.args[1])
-                end
+              end
+
+              return_check = if ret.equal?(self)
+                type.args[1].is_a?(Types::Bases::Bottom)
+              else
+                value(ret, type.args[1])
+              end
+
+              value_check && return_check
             end
           else
             Test.call(val, IS_AP, klass)
diff --git a/test/rbs/test/type_check_test.rb b/test/rbs/test/type_check_test.rb
index 2d709ffd7..40c852dfa 100644
--- a/test/rbs/test/type_check_test.rb
+++ b/test/rbs/test/type_check_test.rb
@@ -60,6 +60,8 @@ def to_int: () -> Integer
 
         assert typecheck.value([1,2,3].each, parse_type("Enumerator[Integer, Array[Integer]]"))
         assert typecheck.value(loop, parse_type("Enumerator[nil, bot]"))
+        assert typecheck.value([].each, parse_type("Enumerator[String, Array[String]]"))
+        assert typecheck.value([].each, parse_type("Enumerator[Integer, Array[Integer]]"))
 
         assert typecheck.value(true, parse_type("bool"))
         assert typecheck.value(false, parse_type("bool"))
diff --git a/test/stdlib/String_test.rb b/test/stdlib/String_test.rb
index dcc4ec74d..44f908248 100644
--- a/test/stdlib/String_test.rb
+++ b/test/stdlib/String_test.rb
@@ -959,8 +959,6 @@ def test_gsub
   end
 
   def test_gsub!
-    omit 'There is currently a bug that prevents `.gsub!` from being testable'
-
     with_string('l').and /l/ do |pattern|
       assert_send_type  '(Regexp | string) -> Enumerator[String, String]',
                         +'hello', :gsub!, pattern

From 812b73af1595b9c136e2efef361a6405e3a4c5d0 Mon Sep 17 00:00:00 2001
From: ksss <co000ri@gmail.com>
Date: Mon, 18 Mar 2024 18:48:59 +0900
Subject: [PATCH 3/3] Fix not match case

---
 test/stdlib/String_test.rb | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/test/stdlib/String_test.rb b/test/stdlib/String_test.rb
index 44f908248..fc740ce06 100644
--- a/test/stdlib/String_test.rb
+++ b/test/stdlib/String_test.rb
@@ -962,7 +962,7 @@ def test_gsub!
     with_string('l').and /l/ do |pattern|
       assert_send_type  '(Regexp | string) -> Enumerator[String, String]',
                         +'hello', :gsub!, pattern
-      assert_send_type  '(Regexp | string) -> Enumerator[String, String]',
+      assert_send_type  '(Regexp | string) -> Enumerator[String, nil]',
                         +'heya', :gsub!, pattern
 
       assert_send_type  '(Regexp | string) { (String) -> _ToS } -> String',