Generic type variable is not solved when the @param type is a union (generic<A>, nil)

[apiology]

Claude: Written by Claude and posted using @apiology's GitHub credentials, while auditing @sg-ignore usage in a downstream project.

When a @param type is a union containing generic<A>, A isn't solved against the union — it appears to bind to the whole argument type, so the return stays a union and every downstream call fails. sorbet-runtime's T.must is this exact shape. The trigger is the union, not nil — @param arg [generic<A>, Integer] fails the same way, while a bare @param arg [generic<A>] binds correctly.

# @generic A
# @param arg [generic<A>, nil]
# @return [generic<A>]
def must_nilable(arg)
  raise if arg.nil?

  arg
end

# @param arg [String, nil]
# @return [Integer]
def via_nilable(arg) = must_nilable(arg).length

$ solargraph typecheck --level strong repro.rb
repro.rb:7: Declared return type generic<A> does not match inferred type generic<A>, nil for #must_nilable
repro.rb:15: #via_nilable return type could not be inferred
repro.rb:15: Unresolved call to length on String, nil
3 problems found.

Expected 0 problems. The two line-15 errors are this bug. The line-7 error is #1276 (raise doesn't narrow), not this — an identical helper with no generics at all (@param arg [String, nil] / @return [String]) produces the same complaint, so it's independent.

Possibly the argument-side counterpart of #1272; not fixed by #1273.

Environment: castwide/solargraph master 8fda63384, rbs 4.0.3, Ruby 3.3.12.