KT-2739 Error type inferred for hashSet(Pair, Pair, Pair)

#KT-2739 Fixed

compiler/testData/diagnostics/tests/inference/varargs/varargsAndPair.kt

@@ -0,0 +1,8 @@
+fun foo<T>(vararg <!UNUSED_PARAMETER!>ts<!>: T): T? = null
+
+class Pair<A>(a: A)
+
+fun test() {
+    val v = foo(Pair(1))
+    <!TYPE_MISMATCH!>v<!>: Int // check that it is not error type
+}