From b6aff98fd7fbb78532de718ff561f26fb7c43c61 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Sun, 17 Mar 2019 17:39:08 -1000
Subject: [PATCH] Add test relating type predicates with and without this
 parameters

---
 .../contravariantInferenceAndTypeGuard.ts     | 27 +++++++++++++++++++
 1 file changed, 27 insertions(+)
 create mode 100644 tests/cases/compiler/contravariantInferenceAndTypeGuard.ts

diff --git a/tests/cases/compiler/contravariantInferenceAndTypeGuard.ts b/tests/cases/compiler/contravariantInferenceAndTypeGuard.ts
new file mode 100644
index 00000000000..a1bcf4ae25d
--- /dev/null
+++ b/tests/cases/compiler/contravariantInferenceAndTypeGuard.ts
@@ -0,0 +1,27 @@
+// @strict: true
+interface ListItem<TData> {
+    prev: ListItem<TData> | null;
+    next: ListItem<TData> | null;
+    data: TData;
+}
+type IteratorFn<TData, TResult, TContext = List<TData>> = (this: TContext, item: TData, node: ListItem<TData>, list: List<TData>) => TResult;
+type FilterFn<TData, TResult extends TData, TContext = List<TData>> = (this: TContext, item: TData, node: ListItem<TData>, list: List<TData>) => item is TResult;
+
+declare class List<TData> {
+    filter<TContext, TResult extends TData>(fn: FilterFn<TData, TResult, TContext>, context: TContext): List<TResult>;
+    filter<TResult extends TData>(fn: FilterFn<TData, TResult>): List<TResult>;
+    filter<TContext>(fn: IteratorFn<TData, boolean, TContext>, context: TContext): List<TData>;
+    filter(fn: IteratorFn<TData, boolean>): List<TData>;
+}
+interface Test {
+    a: string;
+}
+const list2 = new List<Test | null>();
+const filter1 = list2.filter(function(item, node, list): item is Test {
+    this.b; // $ExpectType string
+    item; // $ExpectType Test | null
+    node; // $ExpectType ListItem<Test | null>
+    list; // $ExpectType List<Test | null>
+    return !!item;
+}, {b: 'c'});
+const x: List<Test> = filter1; // $ExpectType List<Test>