Resolve a self return type per union arm

[apiology]

A method that declares a self return type, called on a union receiver, resolves self against the whole union instead of the arm that supplied the method. Chain::Call#resolve split the binder into arms only to collect method pins, then flattened them and called #inferred_pins once with name_pin still bound to the whole union. Each arm's pin is now resolved against that arm alone.

class Counter
  # @return [self]
  def to_sym
    self
  end
end

# @param x [String, Counter]
# @return [Symbol, Counter]
def probe_union(x)
  x.to_sym
end

before:  probe.rb:10: Declared return type ::Symbol, ::Counter does not match
                      inferred type ::Symbol, ::String, ::Counter for #probe_union
after:   0 problems found.

This covers self in any position ��� @return [Array<self>] on an Alpha, Beta receiver now infers ::Array<::Alpha>, ::Array<::Beta>. Results dedupe on path and resolved return type, so a self-returning method shared by every arm still yields each arm: (String | Symbol)#itself stays String, Symbol.

It also removes an @sg-ignore here: Diagnostics::TypeCheck#diagnose calls level.to_sym where level is String, Symbol, which needed suppressing and now resolves to Symbol.

This PR was written by Claude Code on behalf of @apiology.

[apiology]

Yes ��� YARD reaches this too, and the fix covers it without a second change.

self_to_type runs on the pin's parsed ComplexType, downstream of whichever syntax produced the declaration, so @return [self] and an RBS -> self arrive at the same call in #inferred_pins. Measured rather than reasoned about ��� this file has no RBS in it at all:

class Alpha
  # @return [Symbol]
  def to_thing; end
end

class Beta
  # @return [self]
  def to_thing; end
end

# @param x [Alpha, Beta]
# @return [Symbol, Beta]
def probe_union(x)
  x.to_thing
end

solargraph typecheck --level strong, master at 8fda63384:

probe.rb:17: Declared return type ::Symbol, ::Beta does not match inferred type ::Symbol, ::Alpha, ::Beta for #probe_union
1 problem found.

Same file at 4931a9554: 0 problems found.

Two further shapes, both pure YARD, inferred type before and after:

shape	master	branch
@return [self] inherited by both arms	::Alpha, ::Beta	::Alpha, ::Beta
@return [Array<self>] inherited by both arms	::Array<::Alpha, ::Beta>	::Array<::Alpha>, ::Array<::Beta>

The first is the dedup guard holding ��� a method shared by every arm is not over-split. The second is the generic case distributing.

This comment was written by Claude Code on behalf of @apiology.