Add tests for nullable loop variable in for-loop over unsigned progression.

compiler/testData/codegen/box/ranges/unsigned/nullableLoopParameter/progressionExpression.kt

@@ -0,0 +1,18 @@
+// IGNORE_BACKEND: JVM
+// See KT-38833: Runtime exception is "java.lang.ClassCastException: java.lang.Integer cannot be cast to kotlin.UInt"
+
+// WITH_RUNTIME
+// KJS_WITH_FULL_RUNTIME
+
+fun box(): String {
+    var result = 0u
+    val uIntRange: UIntProgression = 1u..3u
+    for (i: UInt? in uIntRange) {
+        result = sum(result, i)
+    }
+    return if (result == 6u) "OK" else "fail: $result"
+}
+
+fun sum(i: UInt, z: UInt?): UInt {
+    return i + z!!
+}

compiler/testData/codegen/box/ranges/unsigned/nullableLoopParameter/rangeExpression.kt

@@ -0,0 +1,18 @@
+// IGNORE_BACKEND: JVM
+// See KT-38833: Runtime exception is "java.lang.ClassCastException: java.lang.Integer cannot be cast to kotlin.UInt"
+
+// WITH_RUNTIME
+// KJS_WITH_FULL_RUNTIME
+
+fun box(): String {
+    var result = 0u
+    val uIntRange = 1u..3u
+    for (i: UInt? in uIntRange) {
+        result = sum(result, i)
+    }
+    return if (result == 6u) "OK" else "fail: $result"
+}
+
+fun sum(i: UInt, z: UInt?): UInt {
+    return i + z!!
+}

compiler/testData/codegen/box/ranges/unsigned/nullableLoopParameter/rangeLiteral.kt

@@ -0,0 +1,17 @@
+// IGNORE_BACKEND: JVM
+// See KT-38833: Runtime exception is "java.lang.ClassCastException: java.lang.Integer cannot be cast to kotlin.UInt"
+
+// WITH_RUNTIME
+// KJS_WITH_FULL_RUNTIME
+
+fun box(): String {
+    var result = 0u
+    for (i: UInt? in 1u..3u) {
+        result = sum(result, i)
+    }
+    return if (result == 6u) "OK" else "fail: $result"
+}
+
+fun sum(i: UInt, z: UInt?): UInt {
+    return i + z!!
+}