Add error for asserting non-nullness of already non-null expressions

Sheeo · Sheeo · commit e6d1651e4d28 · 2017-01-30T15:25:11.000+01:00
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
@@ -13970,7 +13970,18 @@ namespace ts {
         }
 
         function checkNonNullAssertion(node: NonNullExpression) {
-            return getNonNullableType(checkExpression(node.expression));
+            if(!strictNullChecks) {
+                // Actually adding this error results in numerous errors compiling the ts codebase
+                // In the interest of keeping this PR small, it's been omitted
+                // error(node, Diagnostics.Use_of_non_null_assertion_operator_without_strictNullChecks_enabled);
+                return checkExpression(node.expression);
+            }
+            const expr = checkExpression(node.expression);
+            const nonNullable = getNonNullableType(expr);
+            if(isTypeIdenticalTo(nonNullable, expr)) {
+                error(node, Diagnostics.The_expression_asserted_to_be_non_null_is_already_non_null);
+            }
+            return nonNullable;
         }
 
         function checkMetaProperty(node: MetaProperty) {
diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
@@ -2043,6 +2043,14 @@
         "category": "Error",
         "code": 2704
     },
+    "The expression asserted to be non-null is already non-null": {
+        "category": "Error",
+        "code": 2705
+    },
+    "Use of non-null assertion operator without strictNullChecks enabled": {
+        "category": "Error",
+        "code": 2706
+    },
 
     "Import declaration '{0}' is using private name '{1}'.": {
         "category": "Error",
