Disallow type predicate as type assertion

src/compiler/checker.ts

@@ -9656,16 +9656,22 @@ namespace ts {
         function checkAssertion(node: AssertionExpression) {
             const exprType = getRegularTypeOfObjectLiteral(checkExpression(node.expression));
             const targetType = getTypeFromTypeNode(node.type);
-            if (produceDiagnostics && targetType !== unknownType) {
-                const widenedType = getWidenedType(exprType);
+            if (produceDiagnostics) {
+                if (node.type.kind === SyntaxKind.TypePredicate) {
+                    checkTypePredicate(<TypePredicateNode>node.type);
+                }
+                else if (targetType !== unknownType) {
+                    const widenedType = getWidenedType(exprType);
 
-                // Permit 'number[] | "foo"' to be asserted to 'string'.
-                const bothAreStringLike =
-                    someConstituentTypeHasKind(targetType, TypeFlags.StringLike) &&
+                    // Permit 'number[] | "foo"' to be asserted to 'string'.
+                    const bothAreStringLike =
+                        someConstituentTypeHasKind(targetType, TypeFlags.StringLike) &&
                         someConstituentTypeHasKind(widenedType, TypeFlags.StringLike);
-                if (!bothAreStringLike && !(isTypeAssignableTo(targetType, widenedType))) {
-                    checkTypeAssignableTo(exprType, targetType, node, Diagnostics.Neither_type_0_nor_type_1_is_assignable_to_the_other);
+                    if (!bothAreStringLike && !(isTypeAssignableTo(targetType, widenedType))) {
+                        checkTypeAssignableTo(exprType, targetType, node, Diagnostics.Neither_type_0_nor_type_1_is_assignable_to_the_other);
+                    }
                 }
+
             }
             return targetType;
         }

tests/baselines/reference/typeAssertions.errors.txt

@@ -7,9 +7,13 @@ tests/cases/conformance/expressions/typeAssertions/typeAssertions.ts(37,13): err
   Property 'q' is missing in type 'SomeDerived'.
 tests/cases/conformance/expressions/typeAssertions/typeAssertions.ts(38,13): error TS2352: Neither type 'SomeBase' nor type 'SomeOther' is assignable to the other.
   Property 'q' is missing in type 'SomeBase'.
+tests/cases/conformance/expressions/typeAssertions/typeAssertions.ts(44,5): error TS1228: A type predicate is only allowed in return type position for functions and methods.
+tests/cases/conformance/expressions/typeAssertions/typeAssertions.ts(45,2): error TS2322: Type 'number | string' is not assignable to type 'string'.
+  Type 'number' is not assignable to type 'string'.
+tests/cases/conformance/expressions/typeAssertions/typeAssertions.ts(48,32): error TS1228: A type predicate is only allowed in return type position for functions and methods.
 
 
-==== tests/cases/conformance/expressions/typeAssertions/typeAssertions.ts (5 errors) ====
+==== tests/cases/conformance/expressions/typeAssertions/typeAssertions.ts (8 errors) ====
     // Function call whose argument is a 1 arg generic function call with explicit type arguments
     function fn1<T>(t: T) { }
     function fn2(t: any) { }
@@ -64,5 +68,21 @@ tests/cases/conformance/expressions/typeAssertions/typeAssertions.ts(38,13): err
 !!! error TS2352:   Property 'q' is missing in type 'SomeBase'.
     someOther = <SomeOther>someOther;
 
+    // Type assertion cannot be a type-predicate type
+    var numOrStr: number | string;
+    var str: string;
+    if(<numOrStr is string>(numOrStr === undefined)) { // Error
+        ~~~~~~~~~~~~~~~~~~
+!!! error TS1228: A type predicate is only allowed in return type position for functions and methods.
+    	str = numOrStr; // Error, no narrowing occurred
+    	~~~
+!!! error TS2322: Type 'number | string' is not assignable to type 'string'.
+!!! error TS2322:   Type 'number' is not assignable to type 'string'.
+    }
+
+    if((numOrStr === undefined) as numOrStr is string) { // Error
+                                   ~~~~~~~~~~~~~~~~~~
+!!! error TS1228: A type predicate is only allowed in return type position for functions and methods.
+    }
 
 

tests/baselines/reference/typeAssertions.js

@@ -39,6 +39,15 @@ someOther = <SomeOther>someDerived; // Error
 someOther = <SomeOther>someBase; // Error
 someOther = <SomeOther>someOther;
 
+// Type assertion cannot be a type-predicate type
+var numOrStr: number | string;
+var str: string;
+if(<numOrStr is string>(numOrStr === undefined)) { // Error
+	str = numOrStr; // Error, no narrowing occurred
+}
+
+if((numOrStr === undefined) as numOrStr is string) { // Error
+}
 
 
 
@@ -87,3 +96,11 @@ someDerived = someOther; // Error
 someOther = someDerived; // Error
 someOther = someBase; // Error
 someOther = someOther;
+// Type assertion cannot be a type-predicate type
+var numOrStr;
+var str;
+if ((numOrStr === undefined)) {
+    str = numOrStr; // Error, no narrowing occurred
+}
+if ((numOrStr === undefined)) {
+}

tests/cases/conformance/expressions/typeAssertions/typeAssertions.ts

@@ -38,4 +38,13 @@ someOther = <SomeOther>someDerived; // Error
 someOther = <SomeOther>someBase; // Error
 someOther = <SomeOther>someOther;
 
+// Type assertion cannot be a type-predicate type
+var numOrStr: number | string;
+var str: string;
+if(<numOrStr is string>(numOrStr === undefined)) { // Error
+	str = numOrStr; // Error, no narrowing occurred
+}
+
+if((numOrStr === undefined) as numOrStr is string) { // Error
+}