Allow null keyword in a type position

src/rules/noNullKeywordRule.ts

@@ -31,8 +31,18 @@ export class Rule extends Lint.Rules.AbstractRule {
 class NullWalker extends Lint.RuleWalker {
     public visitNode(node: ts.Node) {
         super.visitNode(node);
-        if (node.kind === ts.SyntaxKind.NullKeyword) {
+        if (node.kind === ts.SyntaxKind.NullKeyword && !isPartOfType(node)) {
             this.addFailure(this.createFailure(node.getStart(), node.getWidth(), Rule.FAILURE_STRING));
         }
     }
 }
+
+function isPartOfType({ parent }: ts.Node) {
+    while (parent != null) {
+        if (ts.SyntaxKind.FirstTypeNode <= parent.kind && parent.kind <= ts.SyntaxKind.LastTypeNode) {
+            return true;
+        }
+        parent = parent.parent;
+    }
+    return false;
+}

test/rules/no-null-keyword/test.ts.lint

@@ -2,3 +2,8 @@ var x = null; // error
         ~~~~           [Use 'undefined' instead of 'null']
 console.log(null, x); // error
             ~~~~               [Use 'undefined' instead of 'null']
+
+let match(): string | null;
+interface foo {
+    bar: [number, null, string];
+}