Bidirectional typing on true/false

They may have type of `true` and `false`.

lib/steep/type_construction.rb

@@ -1094,7 +1094,13 @@ def synthesize(node, hint: nil)
           end
 
         when :true, :false
-          add_typing(node, type: AST::Types::Boolean.new)
+          ty = node.type == :true ? AST::Types::Literal.new(value: true) : AST::Types::Literal.new(value: false)
+
+          if hint && check_relation(sub_type: ty, super_type: hint).success?
+            add_typing(node, type: hint)
+          else
+            add_typing(node, type: AST::Types::Boolean.new)
+          end
 
         when :hash
           yield_self do

test/type_construction_test.rb

@@ -2656,6 +2656,26 @@ def test_rescue_bidning_typing
     end
   end
 
+  def test_string_or_true_false
+    with_checker do |checker|
+      source = parse_ruby(<<EOF)
+# @type var x: String | FalseClass
+x = false
+
+# @type var y: String | true
+y = true
+EOF
+
+      with_standard_construction(checker, source) do |construction, typing|
+        _, _, context = construction.synthesize(source.node)
+
+        assert_no_error typing
+        assert_equal parse_type("::String | ::FalseClass"), context.lvar_env[:x]
+        assert_equal parse_type("::String | true"), context.lvar_env[:y]
+      end
+    end
+  end
+
   def test_type_case_case_when
     with_checker do |checker|
       source = parse_ruby(<<EOF)