Assert that ComputedStyleBase is only templated with ComputedStyle

third_party/WebKit/Source/build/scripts/templates/ComputedStyleBase.h.tmpl

 {% from 'macros.tmpl' import license, print_if %}
 {% from 'fields/field.tmpl' import encode, getter_expression, setter_expression, declare_storage, fieldwise_compare, fieldwise_copy, fieldwise_diff, fieldwise_pointer_compare_inherited %}
 {% from 'fields/group.tmpl' import define_field_group_class %}
 {{license()}}
 
 #ifndef ComputedStyleBase_h
 #define ComputedStyleBase_h
 
 #include "core/style/ComputedStyleConstants.h"
 #include "core/CoreExport.h"
@@ skipping 134 matching lines @@
 
   // Fields.
   // TODO(sashab): Remove initialFoo() static methods and update callers to
   // use resetFoo(), which can be more efficient.
 
   {% for field in computed_style.all_fields %}
   // {{field.property_name}}
   {{field_templates[field.field_template].decl_public_methods(field)|indent(2)}}
 
   {% endfor %}
+

[shend, 2017/05/23 22:22:48]

nit: don't need extra space.

[nainar, 2017/05/23 23:45:16]

Done

  private:
   {% for subgroup in computed_style.subgroups %}
   {{define_field_group_class(subgroup)|indent(2)}}
 
   {% endfor %}
 
  protected:
   // Constructor and destructor are protected so that only the parent class ComputedStyle
   // can instantiate this class.
   ALWAYS_INLINE ComputedStyleBase() :
   {% for field in computed_style.fields %}
       {{field.name}}({{encode(field, field.default_value)}}){{print_if(not loop.last, ',')}}
   {% endfor %}
   {
     {% for subgroup in computed_style.subgroups %}
     {{subgroup.member_name}}.Init();
     {% endfor %}
+
+    static_assert(std::is_same<ComputedStyle, ComputedStyleFinal>::value, "ComputedStyleBase can only be templated with ComputedStyle");

[shend, 2017/05/23 22:22:47]

nit: I would move this closer to the top of the class e.g.

template <class T>
class ComputedStyle : public ComputedStyleBase {
  static_assert(...);
};

also wow what magic is this? how does it even know what ComputedStyle is...

[nainar, 2017/05/23 23:45:16]

Done. Since we talked about this IRL. One of the classes imported by
ComputedStyleBase has a forward declaration of ComputedStyle. Makes this build
and work.

+
   }
 
   {% for field in computed_style.all_fields %}
   {% if field.field_template in ('storage_only', 'monotonic_flag', 'external') %}
   // {{field.property_name}}
   {{field_templates[field.field_template].decl_protected_methods(field)|indent(2)}}
 
   {% endif %}
   {% endfor %}
 
@@ skipping 12 matching lines @@
 
  private:
   {% for field in computed_style.fields %}
   {{declare_storage(field)}}
   {% endfor %}
 };
 
 } // namespace blink
 
 #endif // ComputedStyleBase_h