Comment by bhuga

Ruby itself has zero changes from sorbet, so all sorbet syntax has to be valid Ruby. `sig` is implemented as a library.

In this case, your example is not valid syntax, which violates this rule. Not that I personally could tell you why the parser makes a distinction here, but it's at least part of the reason :)

  irb(main):010:0> foo {a: "b"}
  SyntaxError: (irb):10: syntax error, unexpected ':', expecting '}'
  foo {a: "b"}
       ^
  (irb):10: syntax error, unexpected '}', expecting end-of- input
  foo {a: "b"}
            ^
   from /Users/bhuga/.rbenv/versions/2.4/bin/irb:11:in `<main>'
  irb(main):011:0> foo {params(a: "b")}
  NoMethodError: undefined method `foo' for main:Object
   from (irb):11
   from /Users/bhuga/.rbenv/versions/2.4/bin/irb:11:in `<main>'
  irb(main):012:0> 

The `sig` syntax has gone through multiple iterations; within the boundaries of Ruby syntax this is the best we've had.