title: Variadic __is_trivially_constructible built-in predicate
body: <p><a href="http://clang.llvm.org/docs/LanguageExtensions.html#checks-for-type-trait-primitives" rel="nofollow">Documentation of clang</a> WRT built-in type traits says:</p>
  
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    <p>__is_trivially_constructible(type, argtypes...): Determines whether a value of type type can be direct-initialized with arguments of types argtypes... such that no non-trivial functions are called as part of that initialization. This trait is required to implement the C++11 standard library.</p>
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  <p>What is the example of the <em>C++</em> type, which can be non-unary direct-initialized? I.e. (e.g. for arity 2) expression <code>T(x, y)</code> is well-formed and trivial simultaneously?</p>