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Subject: [boost] Help! Detecting that two template aliases are equal.
From: Matt Calabrese (rivorus_at_[hidden])
Date: 2012-11-02 19:26:46

In working on Boost.Generic I need, in multiple parts of the library, to
detect if two template aliases are equivalent. For instance, this is
required when comparing the compatibility of an automatically generated
concept map of a less-refined concept with an explicitly created concept
map for the less-refined concept when the concept in question has an
associated class template. This seems like it shouldn't be too difficult,
but apparently my understanding of template aliases is incomplete:

// Create a template
template< class > struct template_ {};

// Make a simple alias of the template
template< class A > using template_alias = template_< A >;

// Create a metafunction to determine if two unary type templates are the
template< template< class > class L, template< class > class R >
struct templates_are_the_same { static bool constexpr value = false; };

template< template< class > class T >
struct templates_are_the_same< T, T > { static bool constexpr value = true;

// In Clang and GCC, the following static_assert will be triggered
static_assert( templates_are_the_same< template_, template_alias >::value
             , "Templates are not the same."

This behavior is observed in both Clang and GCC, though I haven't verified
that it is correct behavior in the standard. Assuming that it is in fact
standard, can anyone conceive of a way around this limitation? I've
considered passing dummy arguments to the templates based on the template
parameter list and checking that the resultant types are equal, but I'd
like to avoid that if at all possible, since it seems like it would be
difficult or impossible to support certain templates properly (I.E.
consider the template parameter list of a template where one parameter type
is dependent on another, like std::integral_constant). This type of
detection is very important in making Boost.Generic work as closely to
N2914 as possible so any workaround, however hairy, would be a huge help.

-Matt Calabrese

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