class Grape::Validations::Types::PrimitiveCoercer

that it has the proper type.
initialization. When strict is true, it doesn’t coerce a value but check
Coerces the given value to a type defined via a type argument during

def call(val) 

def call(val)
  return InvalidValue.new if reject?(val)
  return nil if val.nil? || treat_as_nil?(val)
  super
end

def initialize(type, strict = false) 

def initialize(type, strict = false)
  super
  @type = type
  @coercer = (strict ? STRICT_MAPPING : MAPPING).fetch(type) do
    scope.const_get(type.name, false)
  rescue NameError
    raise ArgumentError, "type #{type} should support coercion via `[]`" unless type.respond_to?(:[])
    type
  end
end

def reject?(val)

breaking changes.
but Virtus wouldn't accept it. So, this method only exists to not introduce
dry-types is ok to convert an array or a hash to a string, it is supported,
This method maintains logic which was defined by Virtus. For example, 
def reject?(val)
  (val.is_a?(Array) && type == String) ||
    (val.is_a?(String) && type == Hash) ||
    (val.is_a?(Hash) && type == String)
end

def treat_as_nil?(val)

https://github.com/ruby-grape/grape/pull/2045
absence of a value and coerces it into nil. See a discussion there
Dry-Types treats an empty string as invalid. However, Grape considers an empty string as 
def treat_as_nil?(val)
  val == '' && type != String
end