[FIR] Add test cases to prove resolution of backlog tickets

^KT-41728 Fixed
^KT-46586 Fixed
^KT-49696 Fixed
^KT-51160 Fixed
^KT-53308 Fixed

compiler/testData/diagnostics/tests/smartCasts/varnotnull/equalityAndIdentity.kt

@@ -0,0 +1,39 @@
+// !OPT_IN: kotlin.contracts.ExperimentalContracts
+// WITH_STDLIB
+// ISSUE: KT-53308
+
+import kotlin.contracts.*
+
+fun <E> checkEquality(arg: E?) { contract { returns() implies (arg != null) } }
+fun <E> checkIdentity(arg: E?) { contract { returns() implies (arg !== null) } }
+fun checkTrue(statement: Boolean) { contract { returns() implies statement } }
+fun checkFalse(statement: Boolean) { contract { returns() implies !statement } }
+
+fun consume(arg: String) {}
+
+fun test(a: String?) {
+    a.let {
+        checkEquality(it)
+        consume(<!DEBUG_INFO_SMARTCAST!>it<!>)
+    }
+    a.let {
+        checkIdentity(it)
+        consume(<!DEBUG_INFO_SMARTCAST!>it<!>)
+    }
+    a.let {
+        checkTrue(it != null)
+        consume(<!DEBUG_INFO_SMARTCAST!>it<!>)
+    }
+    a.let {
+        checkTrue(it !== null)
+        consume(<!TYPE_MISMATCH!>it<!>)
+    }
+    a.let {
+        checkFalse(it == null)
+        consume(<!DEBUG_INFO_SMARTCAST!>it<!>)
+    }
+    a.let {
+        checkFalse(it === null)
+        consume(<!TYPE_MISMATCH!>it<!>)
+    }
+}