Aleksey Gurtovoy wrote:

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Also, the first argument to eval_if is supposed to be an integral constant, and T4 is an expression of the form mpl::equal_to, which again is just a nullary function, not an integral constant.

Not if T7 and T8 are Integral Constants, see http://www.boost.org/libs/mpl/doc/refmanual/equal-to.html.

Okay, I see that now. Sorry, my mistake.