RBS intersection types (&) parse but are translated to union semantics, not intersection semantics

[apiology]

🤖 Filed by Claude, not the account owner — acting on their behalf via their GitHub credentials.

Summary

RBS type syntax supports intersection types (A & B, see RBS syntax docs), and Solargraph parses & in inline RBS method signatures (#1173) without error. But the semantics aren't implemented: RbsTranslator#type_to_tag converts RBS::Types::Intersection the same way it converts RBS::Types::Union — by joining member types with , :

https://github.com/castwide/solargraph/blob/v0.60.2/lib/solargraph/rbs_translator.rb#L154-L155

when RBS::Types::Intersection
  type.types.map { |member| type_to_tag(member) }.join(', ')

Since Solargraph::ComplexType has no internal representation for intersections (only comma-separated unions), A & B ends up behaving exactly like the union (A, B) once translated — assignable only where every member type would independently be accepted, rather than assignable anywhere any one member type is expected (the actual subtyping rule for intersections: A & B <: A and A & B <: B).

Reproduction

class Consumer
  # @param project_obj [Asana::Resources::Project]
  # @return [void]
  def project_to_h(project_obj); end
end

class MockFactory
  #: () -> (Mocha::Mock & Asana::Resources::Project)
  def make_mock
    Mocha::Mock.new
  end
end

Consumer.new.project_to_h(MockFactory.new.make_mock)

solargraph typecheck --level strong reports:

Wrong argument type for Consumer#project_to_h: project_obj expected Asana::Resources::Project, received Mocha::Mock, Asana::Resources::Project

The "received Mocha::Mock, Asana::Resources::Project" is the union tag — proof the intersection collapsed into a union rather than being checked as a true intersection (which would have passed silently, since the argument's declared type includes Asana::Resources::Project as one conjunct).

Motivating use case

This came up trying to type Minitest+Mocha test doubles precisely: a Mocha::Mock configured via responds_like_instance_of(RealClass) duck-types correctly as RealClass at every call site the code under test exercises, but is statically only a Mocha::Mock. An intersection type would let call sites pass such doubles to methods declared @param x [RealClass] without widening to Object, adding a misleading union, or suppressing with @sg-ignore per call site (see apiology/checkoff#462, e.g. this thread, which hit this ~15-20 times).

What's needed

RbsTranslator (and/or ComplexType) needs an actual intersection representation, distinct from union, so type-compatibility checks honor A & B <: A / A & B <: B rather than flattening to a union tag.