diff --git a/core/io.rbs b/core/io.rbs
index 96f82c1af..3be766eaa 100644
--- a/core/io.rbs
+++ b/core/io.rbs
@@ -1399,7 +1399,8 @@ class IO < Object
   #
   #     AA
   #
-  def putc: (Numeric | String object) -> (Numeric | String)
+  def putc: (real) -> real
+          | (String) -> String
 
   # <!--
   #   rdoc-file=io.c
diff --git a/stdlib/erb/0/erb.rbs b/stdlib/erb/0/erb.rbs
index 7d49f11a2..befb0d3da 100644
--- a/stdlib/erb/0/erb.rbs
+++ b/stdlib/erb/0/erb.rbs
@@ -619,7 +619,7 @@ class ERB
   # It's good practice to choose a variable name that begins with an underscore:
   # <code>'_'</code>.
   #
-  def initialize: (String, ?eoutvar: String, ?trim_mode: Integer | String | NilClass) -> untyped
+  def initialize: (String, ?eoutvar: String, ?trim_mode: Integer | String | NilClass) -> void
 
   # <!-- rdoc-file=lib/erb.rb -->
   # Returns the Ruby code that, when executed, generates the result;
@@ -702,7 +702,7 @@ class ERB
   # Like #result, but prints the result string (instead of returning it);
   # returns `nil`.
   #
-  def run: (?Binding) -> untyped
+  def run: (?Binding) -> nil
 
   # <!--
   #   rdoc-file=lib/erb.rb
@@ -746,7 +746,7 @@ class ERB
   # class MyClass; include MyModule; end
   # MyClass.new.render('foo', 123)                      # => "foo 123"
   #
-  def def_method: (Module, String, ?String) -> untyped
+  def def_method: (Module, String, ?String) -> Symbol
 
   # <!--
   #   rdoc-file=lib/erb.rb
@@ -913,7 +913,7 @@ class ERB
     # define *methodname* as instance method of current module, using ERB object or
     # eRuby file
     #
-    def self.def_erb_method: (String methodname, (String | ERB) erb_or_fname) -> untyped
+    def self.def_erb_method: (String methodname, (String | ERB) erb_or_fname) -> Symbol
   end
 
   # <!-- rdoc-file=lib/erb/util.rb -->
diff --git a/stdlib/monitor/0/monitor.rbs b/stdlib/monitor/0/monitor.rbs
index f4bca5a0e..031775d63 100644
--- a/stdlib/monitor/0/monitor.rbs
+++ b/stdlib/monitor/0/monitor.rbs
@@ -111,7 +111,7 @@ class Monitor
   #   - wait_for_cond(p1, p2)
   # -->
   #
-  def wait_for_cond: (::MonitorMixin::ConditionVariable, Numeric? timeout) -> untyped
+  def wait_for_cond: (::MonitorMixin::ConditionVariable, Numeric? timeout) -> bool
 end
 
 # <!-- rdoc-file=ext/monitor/lib/monitor.rb -->
@@ -334,7 +334,7 @@ class MonitorMixin::ConditionVariable
   # If `timeout` is given, this method returns after `timeout` seconds passed,
   # even if no other thread doesn't signal.
   #
-  def wait: (?Numeric? timeout) -> untyped
+  def wait: (?Numeric? timeout) -> bool
 
   # <!--
   #   rdoc-file=ext/monitor/lib/monitor.rb
@@ -342,7 +342,7 @@ class MonitorMixin::ConditionVariable
   # -->
   # Calls wait repeatedly until the given block yields a truthy value.
   #
-  def wait_until: () { () -> boolish } -> untyped
+  def wait_until: () { () -> boolish } -> nil
 
   # <!--
   #   rdoc-file=ext/monitor/lib/monitor.rb
@@ -350,7 +350,7 @@ class MonitorMixin::ConditionVariable
   # -->
   # Calls wait repeatedly while the given block yields a truthy value.
   #
-  def wait_while: () { () -> boolish } -> untyped
+  def wait_while: () { () -> boolish } -> nil
 
   private
 
diff --git a/stdlib/stringio/0/stringio.rbs b/stdlib/stringio/0/stringio.rbs
index f7a8ffaf0..34cd3a9b5 100644
--- a/stdlib/stringio/0/stringio.rbs
+++ b/stdlib/stringio/0/stringio.rbs
@@ -1271,7 +1271,8 @@ class StringIO
   # -->
   # See IO#putc.
   #
-  def putc: (Numeric | String arg0) -> untyped
+  def putc: (real) -> real
+          | (String) -> String
 
   def puts: (*untyped arg0) -> nil
 
diff --git a/stdlib/zlib/0/gzip_file.rbs b/stdlib/zlib/0/gzip_file.rbs
index 5da64df3d..66b79e483 100644
--- a/stdlib/zlib/0/gzip_file.rbs
+++ b/stdlib/zlib/0/gzip_file.rbs
@@ -143,7 +143,7 @@ module Zlib
     # `flush` method.  While `sync` mode is `true`, the compression ratio decreases
     # sharply.
     #
-    def sync=: (boolish) -> untyped
+    def sync=: (boolish) -> boolish
 
     # <!--
     #   rdoc-file=ext/zlib/zlib.c
diff --git a/stdlib/zlib/0/gzip_writer.rbs b/stdlib/zlib/0/gzip_writer.rbs
index cbcc6af57..c96818316 100644
--- a/stdlib/zlib/0/gzip_writer.rbs
+++ b/stdlib/zlib/0/gzip_writer.rbs
@@ -125,7 +125,8 @@ module Zlib
     # -->
     # Same as IO.
     #
-    def putc: (Numeric | String arg0) -> untyped
+    def putc: (real) -> real
+            | (String) -> String
 
     # <!--
     #   rdoc-file=ext/zlib/zlib.c