how to make a simple generic function... parameterized by collection & value

This is a followup to a mailing with the same subject. I had v.
helpful replies from wasti.r...@gmx.net and Daniel Kr?gler and have
now progressed to this:

template<typename T>
using Validator = std::function<T(*)(const std::string&)>;

template<typename C>
Validator<typename C::value_type>
makeSetValidator(const C &validItems,
                 Validator<typename C::value_type> validate)
{
    return [=](const std::string &s)->typename C::value_type{
        const auto &x = validate(s); // returns valid item of correct
type or throws
        if (std::find(std::begin(validItems), std::end(validItems),
x))
            return x;
        throw ValueError("Invalid item '" + s + "'");
    };
}

The purpose of the code is to be able to supply a collection of valid
items and a per-item validation function and to get back a new
validation function. The returned function is expected to be called
with an item which must then be validated in two ways (1) by calling
the captured validate() function and (2) by ensuring that the item is
in the collection of valid items.

Unfortunately, although this compiles, it doesn't work because
compilation fails at the call site. For example:

auto v = makeSetValidator<int>(std::set<int>{-4, 8, 31},
validate<int>);

The validate() function is a template function:

template<typename T>
T validate(const std::string &s) { ... }

g++ 4.7.0 says:

main.cpp: In function ?void test9(const string&)?:
main.cpp:318:54: error: no matching function for call to
?makeSetValidator(std::set<int>, <unresolved overloaded function
type>)?
main.cpp:318:54: note: candidate is:
In file included from optarg_option.hpp:17:0,
                 from optarg.hpp:33,
                 from main.cpp:14:
optarg_validator.hpp:98:1: note: template<class C>
OptArg::Validator<typename C::value_type>
OptArg::makeSetValidator(const C&, OptArg::Validator<typename
C::value_type>)
optarg_validator.hpp:98:1: note: template argument deduction/
substitution failed:
optarg_validator.hpp: In substitution of ?template<class C>
OptArg::Validator<typename C::value_type>
OptArg::makeSetValidator(const C&, OptArg::Validator<typename
C::value_type>) [with C = int]?:
main.cpp:318:54: required from here
optarg_validator.hpp:94:58: error: ?int? is not a class, struct, or
union type
optarg_validator.hpp:94:58: error: ?int? is not a class, struct, or
union type

Note that it also fails if I provide a set<std::string> and use
validate<std::string> as the validator function.

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