[boost] format lib patch

From: François Dumont (francois.cppdevs_at_[hidden])
Date: 2005-01-17 01:46:48

Config: STLport 5.0 beta and MSVC 8 (but also reported with 7.1)

    It looks like this compiler has a problem with the relation between the 
basic_string::size_type and basic_format::size_type as soon as the library 
is not the native one. Here is a patch proposition that return a 
basic_string::size_type rather than the basic_format::size_type. I think it 
is quite a secure patch as the method is already using a 
static_cast<typename string_type::size_type> to return its result and not a 
simple static_cast<size_type>.

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