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DCL50-CPP-EX2: The declaration of C-style variadic functions that are never defined is permitted, as it not harmful and can be useful in un-evaluated contexts. When a function call expression appears in an unevaluated context, such as the argument in a sizeof expression, overload resolution is performed to determine the result type of the call but does not require a function definition. Some template metaprogramming techniques that employ "substitution failure is not an error" (SFINAE) use variadic functions to implement compile-time type queries, as in the following:
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