code: func (st *state) bareFunctionType(hasReturnType bool) AST {
  	if len(st.str) > 0 && st.str[0] == 'J' {
  		hasReturnType = true
  		st.advance(1)
  	}
  	var returnType AST
  	if hasReturnType {
  		returnType = st.demangleType(false)
  	}
  	types := st.parmlist()
  	return &FunctionType{Return: returnType, Args: types}
  }
docs: // <bare-function-type> ::= [J]<type>+