Fix #partition_union

lib/steep/ast/types/factory.rb

@@ -358,16 +358,21 @@ def partition_union(type)
               partition_union(unfold)
             end
           when AST::Types::Union
-            falsy_types, truthy_types = type.types.partition do |type|
-              (type.is_a?(AST::Types::Literal) && type.value == false) ||
-                type.is_a?(AST::Types::Nil)
+            truthy_types = [] #: Array[AST::Types::t]
+            falsy_types = [] #: Array[AST::Types::t]
+
+            type.types.each do |type|
+              truthy, falsy = partition_union(type)
+
+              truthy_types << truthy if truthy
+              falsy_types << falsy if falsy
             end
 
             [
               truthy_types.empty? ? nil : AST::Types::Union.build(types: truthy_types),
               falsy_types.empty? ? nil : AST::Types::Union.build(types: falsy_types)
             ]
-          when AST::Types::Any, AST::Types::Boolean
+          when AST::Types::Any, AST::Types::Boolean, AST::Types::Top, AST::Types::Logic::Base
             [type, type]
           when AST::Types::Nil
             [nil, type]

test/type_check_test.rb

@@ -841,4 +841,28 @@ def test_type_case__local_variable_narrowing
       assert_equal "::foo", typing.type_of(node: node).to_s
     end
   end
+
+  def test_branch_unreachable__logic_type
+    run_type_check_test(
+      signatures: {
+      },
+      code: {
+        "a.rb" => <<~RUBY
+          x = 1
+          y = x.is_a?(String)
+
+          if y
+            z = 1
+          else
+            z = 2
+          end
+        RUBY
+      },
+      expectations: <<~YAML
+        ---
+        - file: a.rb
+          diagnostics: []
+      YAML
+    )
+  end
 end

test/type_factory_test.rb

@@ -395,4 +395,31 @@ class Bar
       end
     end
   end
+
+  def test_partition_union__top
+    with_factory() do |factory|
+      factory.partition_union(factory.type(parse_type("top"))).tap do |truthy, falsy|
+        assert_equal factory.type(parse_type("top")), truthy
+        assert_equal factory.type(parse_type("top")), falsy
+      end
+    end
+  end
+
+  def test_partition_union__boolish
+    with_factory() do |factory|
+      factory.partition_union(factory.type(parse_type("::boolish"))).tap do |truthy, falsy|
+        assert_equal factory.type(parse_type("top")), truthy
+        assert_equal factory.type(parse_type("top")), falsy
+      end
+    end
+  end
+
+  def test_partition_union__bool_union
+    with_factory() do |factory|
+      factory.partition_union(factory.type(parse_type("bool | ::Symbol"))).tap do |truthy, falsy|
+        assert_equal factory.type(parse_type("bool | ::Symbol")), truthy
+        assert_equal factory.type(parse_type("bool")), falsy
+      end
+    end
+  end
 end