formula: don't expand unused optional dependencies

This properly addresses Homebrew/homebrew-core#7826.

Library/Homebrew/formula.rb

@@ -1505,7 +1505,12 @@ class Formula
   # Returns a list of Dependency objects that are required at runtime.
   # @private
   def runtime_dependencies
-    recursive_dependencies { |_, dep| Dependency.prune if dep.build? }
+    recursive_dependencies do |_dependent, dependency|
+      Dependency.prune if dependency.build?
+      if dependency.optional? || dependency.recommended?
+        Dependency.prune unless build.with?(dependency)
+      end
+    end
   end
 
   # Returns a list of formulae depended on by this formula that aren't

Library/Homebrew/test/formula_test.rb

@@ -666,6 +666,29 @@ class FormulaTests < Homebrew::TestCase
     assert_equal %w[f1 f4], f5.runtime_dependencies.map(&:name)
   end
 
+  def test_runtime_dependencies_with_optional_deps_from_tap
+    tap_loader = mock
+    tap_loader.stubs(:get_formula).raises(RuntimeError, "tried resolving tap formula")
+    Formulary.stubs(:loader_for).with("foo/bar/f1", from: nil).returns(tap_loader)
+
+    stub_formula_loader formula("f2") { url "f2-1.0" }, "baz/qux/f2"
+
+    f3 = formula("f3") do
+      url "f3-1.0"
+      depends_on "foo/bar/f1" => :optional
+      depends_on "baz/qux/f2"
+    end
+
+    # f1 shouldn't be loaded by default.
+    # If it is, an exception will be raised.
+    assert_equal %w[baz/qux/f2], f3.runtime_dependencies.map(&:name)
+
+    # If --with-f1, f1 should be loaded.
+    stub_formula_loader formula("f1") { url "f1-1.0" }, "foo/bar/f1"
+    f3.build = BuildOptions.new(Options.create(%w[--with-f1]), f3.options)
+    assert_equal %w[foo/bar/f1 baz/qux/f2], f3.runtime_dependencies.map(&:name)
+  end
+
   def test_to_hash
     f1 = formula("foo") do
       url "foo-1.0"