Check that subject expression is evaluated only once

compiler/testData/codegen/box/when/whenSubjectVariable/subjectExpressionIsEvaluatedOnce.kt

@@ -0,0 +1,22 @@
+// !LANGUAGE: +VariableDeclarationInWhenSubject
+// IGNORE_BACKEND: JS
+
+var effectCount = 0
+
+fun withSideEffect(): Any {
+    effectCount++
+    return 42
+}
+
+fun box(): String {
+    when (val y = withSideEffect()) {
+        1 -> throw AssertionError()
+        "" -> throw AssertionError()
+        is String -> throw AssertionError()
+        42 -> {}
+    }
+
+    if (effectCount != 1) throw AssertionError("effectCount=$effectCount")
+
+    return "OK"
+}