linkage: add --strict flag to detect opportunistic linkage

There was a previous discussion about making `brew linkage --test` fail
for unrequested dependencies (#9172). I'm not sure what the outcome of
that was, but it still seems like a good idea to try to help us find
cases of opportunistic linkage as they happen rather than when they
cause CI failures in another PR sometime later.

Let's do this by adding a `--strict` flag to `brew linkage --test`. My
intention is for `brew linkage --test --strict` failures to be warnings
rather than errors in CI, which should mitigate some of the concerns
about doing this that were raised in #9172.

Library/Homebrew/cli/args.rbi

@@ -3,6 +3,9 @@
 module Homebrew
   module CLI
     class Args < OpenStruct
+      sig { returns(T::Boolean) }
+      def strict?; end
+
       sig { returns(T::Boolean) }
       def HEAD?; end
 

Library/Homebrew/dev-cmd/linkage.rb

@@ -20,6 +20,9 @@ def linkage_args
       switch "--test",
              description: "Show only missing libraries and exit with a non-zero status if any missing "\
                           "libraries are found."
+      switch "--strict",
+             depends_on:  "--test",
+             description: "Exit with a non-zero status if any undeclared dependencies with linkage are found."
       switch "--reverse",
              description: "For every library that a keg references, print its dylib path followed by the "\
                           "binaries that link to it."
@@ -46,8 +49,8 @@ def linkage
         result = LinkageChecker.new(keg, cache_db: db)
 
         if args.test?
-          result.display_test_output
-          Homebrew.failed = true if result.broken_library_linkage?
+          result.display_test_output(strict: args.strict?)
+          Homebrew.failed = true if result.broken_library_linkage?(strict: args.strict?)
         elsif args.reverse?
           result.display_reverse_output
         else

Library/Homebrew/linkage_checker.rb

@@ -62,17 +62,19 @@ def display_reverse_output
     end
   end
 
-  def display_test_output(puts_output: true)
+  def display_test_output(puts_output: true, strict: false)
     display_items "Missing libraries", broken_dylibs_with_expectations, puts_output: puts_output
     display_items "Unused missing linkage information", unexpected_present_dylibs, puts_output: puts_output
     display_items "Broken dependencies", @broken_deps, puts_output: puts_output
     display_items "Unwanted system libraries", @unwanted_system_dylibs, puts_output: puts_output
     display_items "Conflicting libraries", @version_conflict_deps, puts_output: puts_output
+    display_items "Undeclared dependencies with linkage", @undeclared_deps, puts_output: puts_output if strict
   end
 
-  sig { returns(T::Boolean) }
-  def broken_library_linkage?
+  sig { params(strict: T::Boolean).returns(T::Boolean) }
+  def broken_library_linkage?(strict: false)
     issues = [@broken_deps, @unwanted_system_dylibs, @version_conflict_deps]
+    issues << @undeclared_deps if strict
     [issues, unexpected_broken_dylibs, unexpected_present_dylibs].flatten.any?(&:present?)
   end