Undeclared dependencies in runtime_dependencies

Fixes https://github.com/Homebrew/brew/issues/2173.

Library/Homebrew/formula.rb

@@ -1487,10 +1487,7 @@ class Formula
   # Returns a list of Dependency objects that are required at runtime.
   # @private
   def runtime_dependencies
-    recursive_dependencies do |_, dependency|
-      Dependency.prune if dependency.build?
-      Dependency.prune if !dependency.required? && build.without?(dependency)
-    end
+    declared_runtime_dependencies | undeclared_runtime_dependencies
   end
 
   # Returns a list of formulae depended on by this formula that aren't
@@ -1835,6 +1832,23 @@ class Formula
 
   private
 
+  def declared_runtime_dependencies
+    recursive_dependencies do |_, dependency|
+      Dependency.prune if dependency.build?
+      Dependency.prune if !dependency.required? && build.without?(dependency)
+    end
+  end
+
+  def undeclared_runtime_dependencies
+    return [] unless optlinked?
+
+    keg = Keg.new(opt_prefix)
+    linkage_checker = LinkageChecker.new(keg, self)
+    dylib_formula_names = linkage_checker.brewed_dylibs.keys
+    linked_formulae_names = dylib_formula_names - [name]
+    linked_formulae_names.map { |n| Dependency.new(n) }
+  end
+
   # Returns the prefix for a given formula version number.
   # @private
   def versioned_prefix(v)

Library/Homebrew/test/formula_spec.rb

@@ -685,6 +685,7 @@ describe Formula do
     expect(f5.runtime_dependencies.map(&:name)).to eq(["f1", "f4"])
   end
 
+  describe "#runtime_dependencies" do
     specify "runtime dependencies with optional deps from tap" do
       tap_loader = double
 
@@ -707,6 +708,23 @@ describe Formula do
       expect(f3.runtime_dependencies.map(&:name)).to eq(["foo/bar/f1", "baz/qux/f2"])
     end
 
+    it "includes non-declared direct dependencies" do
+      formula = Class.new(Testball).new
+      dependency = formula("dependency") { url "f-1.0" }
+
+      formula.brew { formula.install }
+      keg = Keg.for(formula.prefix)
+      keg.link
+
+      brewed_dylibs = { dependency.name => Set["some.dylib"] }
+      linkage_checker = double("linkage checker", brewed_dylibs: brewed_dylibs)
+      allow(LinkageChecker).to receive(:new).with(keg, any_args)
+        .and_return(linkage_checker)
+
+      expect(formula.runtime_dependencies).to include an_object_having_attributes(name: dependency.name)
+    end
+  end
+
   specify "requirements" do
     f1 = formula "f1" do
       url "f1-1"