Fix error of prototype rb on hash spread syntax

[pocke]

Fix #464

Cause

Hash key will be a nil on hash spread syntax.
For example

pp RubyVM::AbstractSyntaxTree.parse('{ called: true, **({ foo: foo }.compact) }')

(SCOPE@1:0-1:42
 tbl: []
 args: nil
 body:
   (HASH@1:0-1:42
      (LIST@1:2-1:39 (LIT@1:2-1:9 :called) (TRUE@1:10-1:14) nil # ← here
         (CALL@1:19-1:39
            (HASH@1:19-1:31
               (LIST@1:21-1:29 (LIT@1:21-1:25 :foo) (VCALL@1:26-1:29 :foo)
                  nil)) :compact nil) nil)))

Solution

Determine key and value types as untyped if it a hash has **.

So, the type for the reported code will be the following.

# ruby
def a(foo:)
  { called: true, **({ foo: foo }.compact) }
end

$ exe/rbs prototype rb test.rb
class Object
  def a: (foo: untyped foo) -> ::Hash[:called | untyped, ::TrueClass | untyped]
end

---

By the way, @ybiquitous says "unless .compact, the command passes:".
Because Ruby optimizes a hash spread for a hash literal.

pp RubyVM::AbstractSyntaxTree.parse('{ called: true, **({ foo: foo }) }')

(SCOPE@1:0-1:34
 tbl: []
 args: nil
 body:
   (HASH@1:0-1:34
      (LIST@1:2-1:29 (LIT@1:2-1:9 :called) (TRUE@1:10-1:14)
         (LIT@1:21-1:25 :foo) (VCALL@1:26-1:29 :foo) nil)))

The AST does not have nil as a key.

[pocke]

In RubyVM::AST, all list has trailing nil. It removed the nil by compact previously. But compact removes a nil for hash spread unexpectedly. So I changed compact to pop.

[soutaro]

🎉