Re: [boost] MSVC9 SFINAE quirks?

Subject: Re: [boost] MSVC9 SFINAE quirks?
From: Simonson, Lucanus J (lucanus.j.simonson_at_[hidden])
Date: 2009-01-09 22:20:44

Steven Watanabe wrote:
>  template <typename geometry_type_1, typename geometry_type_2>
>  typename boost::enable_if<
>      boost::mpl::and_<is_any_polygon_set_type<geometry_type_1>,
>                       is_any_polygon_set_type<geometry_type_2>,
>                       is_either_polygon_set_type<geometry_type_1, 
>geometry_type_2>
>      >,
>      polygon_set_view<geometry_type_1, geometry_type_2, 3>
>  >::type
>  operator-(const geometry_type_1& lvalue, const geometry_type_2& rvalue) {
>    return polygon_set_view<geometry_type_1, geometry_type_2, 3>
>      (lvalue, rvalue);
>  }

This won't work.  In order to instantiate enable_if the compiler must instantiate both of its arguments.  That means the compiler will try to instantiate polygon_set_view<> for every time the generic operator - in the stl is used for iterator arithmetic.  Any error encountered trying to instantiate polygon_set_view will cause a syntax error and failed compilation even though the and_ evaluates to false.

To get around this I have added substitution failure when the first parameter is parsed:

  template <typename geometry_type_1, typename geometry_type_2>
  typename requires_1< typename gtl_if<typename gtl_and< 
    typename is_polygon_90_set_type<geometry_type_1>::type,
    typename is_polygon_90_set_type<geometry_type_2>::type>::type>::type,
  polygon_90_set_view<geometry_type_1, geometry_type_2, 		boolean_op::BinaryNot> >::type
  operator-(const geometry_type_1& lvalue, const geometry_type_2& rvalue) {
    return polygon_90_set_view<geometry_type_1, geometry_type_2, 			boolean_op::BinaryNot> 
      (lvalue, rvalue, 
       polygon_90_set_traits<geometry_type_1>::orient(lvalue),
       boolean_op::BinaryNot());
  }
  
requires_1 is equivalent to enable_if, gtl_if returns causes substitution failure unless it receives gtl_yes.  I'm hoping MSVC will accept this syntax; gcc does just fine.

Regards,
Luke