keg: fallback to missing_deps if deps not in tab

Library/Homebrew/keg.rb

@@ -1,6 +1,7 @@
 require "extend/pathname"
 require "keg_relocate"
 require "formula_lock"
+require "diagnostic"
 require "ostruct"
 
 class Keg
@@ -297,14 +298,33 @@ class Keg
   end
 
   def installed_dependents
-    Formula.installed.flat_map(&:installed_kegs).select do |keg|
+    installed_kegs = Formula.installed.flat_map(&:installed_kegs)
-      Tab.for_keg(keg).runtime_dependencies.any? do |dep|
+    installed_kegs_and_tabs = installed_kegs.map { |k| [k, Tab.for_keg(k)] }
+    kegs_by_tab_deps_presence = installed_kegs_and_tabs.group_by do |_, tab|
+      !tab.runtime_dependencies.nil?
+    end
+    [true, false].each { |v| kegs_by_tab_deps_presence[v] ||= [] }
+
+    kegs_by_tab_deps_presence[true].select! do |_, tab|
+      tab.runtime_dependencies.any? do |dep|
         # Resolve formula rather than directly comparing names
         # in case of conflicts between formulae from different taps.
         dep_formula = Formulary.factory(dep["full_name"])
         dep_formula == to_formula && dep["version"] == version.to_s
       end
     end
+
+    remaining_kegs_and_tabs = kegs_by_tab_deps_presence[false]
+    remaining_formulae = remaining_kegs_and_tabs.map { |k, _| k.to_formula }
+
+    # Expensive if installed_dependents of multiple kegs are being checked
+    deps = Homebrew::Diagnostic.missing_deps(remaining_formulae, [name])
+    remaining_kegs_and_tabs.select! do |keg, _|
+      keg_deps = deps[keg.to_formula.full_name]
+      keg_deps && keg_deps.any?
+    end
+
+    kegs_by_tab_deps_presence.values.flatten(1).map { |k, _| k }
   end
 
   def find(*args, &block)

Library/Homebrew/test/test_keg.rb

@@ -339,8 +339,29 @@ class InstalledDependantsTests < LinkTests
     tab.write
   end
 
+  def test_unknown_dependencies
+    dependencies nil
+
+    bar = formula "bar" do
+      url "bar-1.0"
+      depends_on "foo"
+    end
+    stub_formula_loader bar
+
+    assert_equal [@dependent], @keg.installed_dependents
+  end
+
   def test_no_dependencies
     dependencies []
+
+    # Make sure formula dependencies aren't checked when dependencies are
+    # recorded in the tab.
+    bar = formula "bar" do
+      url "bar-1.0"
+      depends_on "foo"
+    end
+    stub_formula_loader bar
+
     assert_empty @keg.installed_dependents
   end