doc of placeholer expression requirements incomplete

[cppljevans]

he 3 expression requirements on:

http://www.boost.org/libs/mpl/doc/refmanual/placeholder-expression.html

make no mention that X should have a nested
type typedef.  IOW, X must be a metafunction
as defined here:

http://www.boost.org/libs/mpl/doc/refmanual/metafunction.html

However, I believe it should since I'm
getting compile-time error messages
without the nested typedef.

[david_abrahams]

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The documentation is correct as is.  std::vector<_1> is a placeholder expression even though std::vector has no nested ::type.

Whatever compiler errors you're seeing are not caused by a bug in the documentation.