Fix UnreachableBranch on == against literal supertype (Symbol / Integer / String)

[rhiroe]

Summary

Fixes a regression introduced by #2101 (shipped in 2.0.0) where comparing a value of type Symbol (or Integer / String) against a literal symbol/integer/string with == is incorrectly reported as Ruby::UnreachableBranch.

Closes #2222

Reproduction

a.rb:

class A
  def foo(symbol)
    if symbol == :qux
      puts "qux"
    else
      puts "other"
    end
  end
end

a.rbs:

class A
  def foo: (Symbol symbol) -> void
end

$ bundle exec steep check --severity-level=hint
a.rb:3:4: [hint] The branch is unreachable
│ Diagnostic ID: Ruby::UnreachableBranch
│
└     if symbol == :qux
      ~~

The same warning was emitted for Integer == 1, String == "x", and the postfix-if form.

Cause

#2101 added for_receiver: true to LogicTypeInterpreter#literal_var_type_case_select so that == against a literal could narrow union members like Result | :some_symbol properly: Result cannot equal :some_symbol, so it should flow only into the falsy branch.

The implementation, however, applied that rule to every non-literal type. Symbol, Integer, and String are non-literal class types but they include the corresponding literal values, so dropping them from the truthy bucket made the if branch look unreachable.

Fix

In for_receiver: true mode, narrow only when the literal type is a subtype of the receiver type:

literal_type = AST::Types::Literal.new(value: value_node.children[0])
if !for_receiver || subtyping?(sub_type: literal_type, super_type: type)
  true_types << literal_type
end
false_types << type

Behavior matrix:

Receiver type vs literal	Truthy	Falsy
Symbol == :qux	:qux	Symbol
Result == :some_symbol (unrelated class)	(empty → unreachable)	Result
(Result | :some_symbol) == :some_symbol	:some_symbol	Result
:foo == :qux (literal vs literal)	(empty → unreachable)	:foo

The first row is the regression being fixed. The remaining rows match #2101's intent and continue to pass smoke/narrowing-else.

[soutaro]

🙏