diff --git a/Library/Homebrew/development_tools.rb b/Library/Homebrew/development_tools.rb
index 3961e14538..fc78720816 100644
--- a/Library/Homebrew/development_tools.rb
+++ b/Library/Homebrew/development_tools.rb
@@ -38,19 +38,6 @@ class DevelopmentTools
       installation_instructions
     end
 
-    # TODO: This method appears to be unused. Can it be deleted?
-    sig { returns(T.nilable(String)) }
-    def default_cc
-      cc = DevelopmentTools.locate "cc"
-      return if cc.nil?
-
-      begin
-        cc.realpath.basename.to_s
-      rescue
-        nil
-      end
-    end
-
     sig { returns(Symbol) }
     def default_compiler
       :clang
@@ -126,7 +113,7 @@ class DevelopmentTools
       {
         "os"         => ENV["HOMEBREW_SYSTEM"],
         "os_version" => OS_VERSION,
-        "cpu_family" => Hardware::CPU.family,
+        "cpu_family" => Hardware::CPU.family.to_s,
       }
     end
     alias generic_build_system_info build_system_info
diff --git a/Library/Homebrew/extend/os/linux/development_tools.rb b/Library/Homebrew/extend/os/linux/development_tools.rb
index 267c9a1516..2648ebca68 100644
--- a/Library/Homebrew/extend/os/linux/development_tools.rb
+++ b/Library/Homebrew/extend/os/linux/development_tools.rb
@@ -21,11 +21,11 @@ class DevelopmentTools
       :gcc
     end
 
-    sig { returns(T::Hash[String, T.untyped]) }
+    sig { returns(T::Hash[String, T.nilable(String)]) }
     def build_system_info
       generic_build_system_info.merge({
-        "glibc_version"     => OS::Linux::Glibc.version,
-        "oldest_cpu_family" => Hardware.oldest_cpu,
+        "glibc_version"     => OS::Linux::Glibc.version.to_s.presence,
+        "oldest_cpu_family" => Hardware.oldest_cpu.to_s,
       })
     end
   end
diff --git a/Library/Homebrew/extend/os/mac/development_tools.rb b/Library/Homebrew/extend/os/mac/development_tools.rb
index 993ba68ba3..353ae30428 100644
--- a/Library/Homebrew/extend/os/mac/development_tools.rb
+++ b/Library/Homebrew/extend/os/mac/development_tools.rb
@@ -64,7 +64,7 @@ class DevelopmentTools
       EOS
     end
 
-    sig { returns(T::Hash[String, T.untyped]) }
+    sig { returns(T::Hash[String, T.nilable(String)]) }
     def build_system_info
       build_info = {
         "xcode"          => MacOS::Xcode.version.to_s.presence,
diff --git a/Library/Homebrew/extend/os/mac/hardware.rb b/Library/Homebrew/extend/os/mac/hardware.rb
index 2b6e8de4ee..43adfa1dea 100644
--- a/Library/Homebrew/extend/os/mac/hardware.rb
+++ b/Library/Homebrew/extend/os/mac/hardware.rb
@@ -4,7 +4,12 @@
 module Hardware
   extend T::Sig
   sig { params(version: T.nilable(Version)).returns(Symbol) }
-  def self.oldest_cpu(version = MacOS.version)
+  def self.oldest_cpu(version = nil)
+    version = if version
+      MacOS::Version.new(version.to_s)
+    else
+      MacOS.version
+    end
     if CPU.arch == :arm64
       :arm_vortex_tempest
     # TODO: this cannot be re-enabled until either Rosetta 2 supports AVX
diff --git a/Library/Homebrew/os/linux/glibc.rb b/Library/Homebrew/os/linux/glibc.rb
index 2d27c1cca1..7478701223 100644
--- a/Library/Homebrew/os/linux/glibc.rb
+++ b/Library/Homebrew/os/linux/glibc.rb
@@ -11,6 +11,7 @@ module OS
 
       module_function
 
+      sig { returns(Version) }
       def system_version
         @system_version ||= begin
           version = Utils.popen_read("/usr/bin/ldd", "--version")[/ (\d+\.\d+)/, 1]
@@ -22,6 +23,7 @@ module OS
         end
       end
 
+      sig { returns(Version) }
       def version
         @version ||= begin
           version = Utils.popen_read(HOMEBREW_PREFIX/"opt/glibc/bin/ldd", "--version")[/ (\d+\.\d+)/, 1]
@@ -38,6 +40,7 @@ module OS
         Version.new(ENV.fetch("HOMEBREW_LINUX_MINIMUM_GLIBC_VERSION"))
       end
 
+      sig { returns(T::Boolean) }
       def below_minimum_version?
         system_version < minimum_version
       end
diff --git a/Library/Homebrew/os/mac.rb b/Library/Homebrew/os/mac.rb
index b3e6cff1ab..47aaaf722a 100644
--- a/Library/Homebrew/os/mac.rb
+++ b/Library/Homebrew/os/mac.rb
@@ -24,16 +24,19 @@ module OS
 
     # This can be compared to numerics, strings, or symbols
     # using the standard Ruby Comparable methods.
+    sig { returns(Version) }
     def version
       @version ||= full_version.strip_patch
     end
 
     # This can be compared to numerics, strings, or symbols
     # using the standard Ruby Comparable methods.
+    sig { returns(Version) }
     def full_version
       @full_version ||= Version.new((ENV["HOMEBREW_MACOS_VERSION"]).chomp)
     end
 
+    sig { params(version: Version).void }
     def full_version=(version)
       @full_version = Version.new(version.chomp)
       @version = nil
diff --git a/Library/Homebrew/os/mac/xcode.rb b/Library/Homebrew/os/mac/xcode.rb
index 9f857364b2..fb36eabcf3 100755
--- a/Library/Homebrew/os/mac/xcode.rb
+++ b/Library/Homebrew/os/mac/xcode.rb
@@ -89,7 +89,7 @@ module OS
 
       # Returns a Pathname object corresponding to Xcode.app's Developer
       # directory or nil if Xcode.app is not installed.
-      sig { returns(Pathname) }
+      sig { returns(T.nilable(Pathname)) }
       def prefix
         @prefix ||=
           begin