Fix incorrect TypeCastException message when casting null

#KT-5121 Fixed

compiler/backend/src/org/jetbrains/kotlin/codegen/ExpressionCodegen.java

@@ -3727,7 +3727,7 @@ The "returned" value of try expression with no finally is either the last expres
 
     @Override
     public StackValue visitBinaryWithTypeRHSExpression(@NotNull JetBinaryExpressionWithTypeRHS expression, StackValue receiver) {
-        final JetExpression left = expression.getLeft();
+        JetExpression left = expression.getLeft();
         final IElementType opToken = expression.getOperationReference().getReferencedNameElementType();
         if (opToken == JetTokens.COLON) {
             return gen(left);
@@ -3752,10 +3752,7 @@ The "returned" value of try expression with no finally is either the last expres
                         v.dup();
                         Label nonnull = new Label();
                         v.ifnonnull(nonnull);
-                        JetType leftType = bindingContext.getType(left);
+                        genThrow(v, "kotlin/TypeCastException", "null cannot be cast to non-null type " +
-                        assert leftType != null;
-                        genThrow(v, "kotlin/TypeCastException", DescriptorRenderer.FQ_NAMES_IN_TYPES.renderType(leftType) +
-                                                                " cannot be cast to " +
                                                                 DescriptorRenderer.FQ_NAMES_IN_TYPES.renderType(rightType));
                         v.mark(nonnull);
                     }

compiler/testData/codegen/boxWithStdlib/regressions/typeCastException.kt

@@ -1,4 +1,7 @@
+import java.util.ArrayList
+
 // KT-2823 TypeCastException has no message
+// KT-5121 Better error message in on casting null to non-null type
 
 fun box(): String {
     try {
@@ -6,9 +9,20 @@ fun box(): String {
         a as Array<String>
     }
     catch (e: TypeCastException) {
-        if (e.getMessage() == "kotlin.Any? cannot be cast to kotlin.Array<kotlin.String>") {
+        if (e.getMessage() != "null cannot be cast to non-null type kotlin.Array<kotlin.String>") {
-            return "OK"
+            return "Fail 1: $e"
         }
     }
-    return "fail"
+
+    try {
+        val x: String? = null
+        x as String
+    }
+    catch (e: TypeCastException) {
+        if (e.getMessage() != "null cannot be cast to non-null type kotlin.String") {
+            return "Fail 2: $e"
+        }
+    }
+
+    return "OK"
 }