do not report cascading errors in instanceof operator

src/compiler/checker.ts

@@ -4935,10 +4935,12 @@ module ts {
             // The instanceof operator requires the left operand to be of type Any, an object type, or a type parameter type,
             // and the right operand to be of type Any or a subtype of the 'Function' interface type. 
             // The result is always of the Boolean primitive type.
-            if (!isTypeAnyTypeObjectTypeOrTypeParameter(leftType)) {
+            // NOTE: do not raise error is leftType is unknown as related error was already reported
+            if (leftType !== unknownType && !isTypeAnyTypeObjectTypeOrTypeParameter(leftType)) {
                 error(node.left, Diagnostics.The_left_hand_side_of_an_instanceof_expression_must_be_of_type_any_an_object_type_or_a_type_parameter);
             }
-            if (rightType !== anyType && !isTypeSubtypeOf(rightType, globalFunctionType)) {
+            // NOTE: do not raise error is right is unknown as related error was already reported
+            if (rightType !== unknownType && rightType !== anyType && !isTypeSubtypeOf(rightType, globalFunctionType)) {
                 error(node.right, Diagnostics.The_right_hand_side_of_an_instanceof_expression_must_be_of_type_any_or_of_a_type_assignable_to_the_Function_interface_type);
             }
             return booleanType;

tests/baselines/reference/constructorWithIncompleteTypeAnnotation.errors.txt

@@ -68,8 +68,6 @@ tests/cases/compiler/constructorWithIncompleteTypeAnnotation.ts(40,28): error TS
 tests/cases/compiler/constructorWithIncompleteTypeAnnotation.ts(41,21): error TS2304: Cannot find name 'retValue'.
 tests/cases/compiler/constructorWithIncompleteTypeAnnotation.ts(47,17): error TS2304: Cannot find name 'console'.
 tests/cases/compiler/constructorWithIncompleteTypeAnnotation.ts(53,13): error TS2304: Cannot find name 'console'.
-tests/cases/compiler/constructorWithIncompleteTypeAnnotation.ts(76,26): error TS2358: The left-hand side of an 'instanceof' expression must be of type 'any', an object type or a type parameter.
-tests/cases/compiler/constructorWithIncompleteTypeAnnotation.ts(76,44): error TS2359: The right-hand side of an 'instanceof' expression must be of type 'any' or of a type assignable to the 'Function' interface type.
 tests/cases/compiler/constructorWithIncompleteTypeAnnotation.ts(89,23): error TS2364: Invalid left-hand side of assignment expression.
 tests/cases/compiler/constructorWithIncompleteTypeAnnotation.ts(108,24): error TS2365: Operator '+' cannot be applied to types 'number' and 'boolean'.
 tests/cases/compiler/constructorWithIncompleteTypeAnnotation.ts(159,31): error TS2304: Cannot find name 'Property'.
@@ -98,7 +96,7 @@ tests/cases/compiler/constructorWithIncompleteTypeAnnotation.ts(259,29): error T
 tests/cases/compiler/constructorWithIncompleteTypeAnnotation.ts(259,37): error TS2304: Cannot find name 'string'.
 
 
-==== tests/cases/compiler/constructorWithIncompleteTypeAnnotation.ts (98 errors) ====
+==== tests/cases/compiler/constructorWithIncompleteTypeAnnotation.ts (96 errors) ====
     declare module "fs" {
         export class File {
             constructor(filename: string);
@@ -242,10 +240,6 @@ tests/cases/compiler/constructorWithIncompleteTypeAnnotation.ts(259,37): error T
     
                 var local5 = <fs.File>null;
                 var local6 = local5 instanceof fs.File;
-                             ~~~~~~
-!!! error TS2358: The left-hand side of an 'instanceof' expression must be of type 'any', an object type or a type parameter.
-                                               ~~~~~~~
-!!! error TS2359: The right-hand side of an 'instanceof' expression must be of type 'any' or of a type assignable to the 'Function' interface type.
     
                 var hex = 0xBADC0DE, Hex = 0XDEADBEEF;
                 var float = 6.02e23, float2 = 6.02E-23

tests/baselines/reference/errorHandlingInInstanceOf.errors.txt

@@ -0,0 +1,15 @@
+tests/cases/compiler/errorHandlingInInstanceOf.ts(1,5): error TS2304: Cannot find name 'x'.
+tests/cases/compiler/errorHandlingInInstanceOf.ts(5,18): error TS2304: Cannot find name 'UnknownType'.
+
+
+==== tests/cases/compiler/errorHandlingInInstanceOf.ts (2 errors) ====
+    if (x instanceof String) {
+        ~
+!!! error TS2304: Cannot find name 'x'.
+    }
+    
+    var y: any;
+    if (y instanceof UnknownType) {
+                     ~~~~~~~~~~~
+!!! error TS2304: Cannot find name 'UnknownType'.
+    }

tests/baselines/reference/errorHandlingInInstanceOf.js

@@ -0,0 +1,14 @@
+//// [errorHandlingInInstanceOf.ts]
+if (x instanceof String) {
+}
+
+var y: any;
+if (y instanceof UnknownType) {
+}
+
+//// [errorHandlingInInstanceOf.js]
+if (x instanceof String) {
+}
+var y;
+if (y instanceof UnknownType) {
+}

tests/cases/compiler/errorHandlingInInstanceOf.ts

@@ -0,0 +1,6 @@
+if (x instanceof String) {
+}
+
+var y: any;
+if (y instanceof UnknownType) {
+}