Actually, it makes more sense to error on the predicate annotation than anything else.

DanielRosenwasser · DanielRosenwasser · commit b191a00b1c7f · 2016-02-12T15:22:14.000-08:00
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
@@ -11567,7 +11567,7 @@ namespace ts {
             const parent = getTypePredicateParent(node);
             if (!parent) {
                 // The parent must not be valid.
-                error(node.parent, Diagnostics.A_type_predicate_is_only_allowed_in_return_type_position_for_functions_and_methods);
+                error(node, Diagnostics.A_type_predicate_is_only_allowed_in_return_type_position_for_functions_and_methods);
                 return;
             }
 

Original file line number	Diff line number	Diff line change
`@@ -11567,7 +11567,7 @@ namespace ts {`
`11567`	`11567`	`const parent = getTypePredicateParent(node);`
`11568`	`11568`	`if (!parent) {`
`11569`	`11569`	`// The parent must not be valid.`
`11570`		`- error(node.parent, Diagnostics.A_type_predicate_is_only_allowed_in_return_type_position_for_functions_and_methods);`
	`11570`	`+ error(node, Diagnostics.A_type_predicate_is_only_allowed_in_return_type_position_for_functions_and_methods);`
`11571`	`11571`	`return;`
`11572`	`11572`	`}`
`11573`	`11573`