From 475036950e768893474093215bbb0c5eaa19ea31 Mon Sep 17 00:00:00 2001
From: Oleksandr T <oleksandr.tarasiuk@outlook.com>
Date: Fri, 12 Feb 2021 03:07:14 +0200
Subject: [PATCH] fix(42133): fix instantiated undefined type from JS
 initializer (#42662)

---
 src/compiler/checker.ts                       |  4 ++--
 .../typeFromJSInitializer.errors.txt          |  7 ++++++
 .../reference/typeFromJSInitializer.symbols   | 19 +++++++++++++++
 .../reference/typeFromJSInitializer.types     | 24 +++++++++++++++++++
 .../salsa/typeFromJSInitializer.ts            |  7 ++++++
 5 files changed, 59 insertions(+), 2 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index d33b94f4285..9d089eb7299 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -19489,8 +19489,8 @@ namespace ts {
         }
 
         function isEmptyArrayLiteralType(type: Type): boolean {
-            const elementType = isArrayType(type) ? getTypeArguments(type)[0] : undefined;
-            return elementType === undefinedWideningType || elementType === implicitNeverType;
+            const elementType = getElementTypeOfArrayType(type);
+            return strictNullChecks ? elementType === implicitNeverType : elementType === undefinedWideningType;
         }
 
         function isTupleLikeType(type: Type): boolean {
diff --git a/tests/baselines/reference/typeFromJSInitializer.errors.txt b/tests/baselines/reference/typeFromJSInitializer.errors.txt
index 08d66a2a9f3..45ed31d6895 100644
--- a/tests/baselines/reference/typeFromJSInitializer.errors.txt
+++ b/tests/baselines/reference/typeFromJSInitializer.errors.txt
@@ -72,4 +72,11 @@ tests/cases/conformance/salsa/a.js(37,5): error TS2322: Type '"error"' is not as
     u = 'ok'
     
     l.push('ok')
+    
+    /** @type {(v: unknown) => v is undefined} */
+    const isUndef = v => v === undefined;
+    const e = [1, undefined];
+    
+    // should be  undefined[]
+    const g = e.filter(isUndef);
     
\ No newline at end of file
diff --git a/tests/baselines/reference/typeFromJSInitializer.symbols b/tests/baselines/reference/typeFromJSInitializer.symbols
index 2de03afed25..98c921e1cc1 100644
--- a/tests/baselines/reference/typeFromJSInitializer.symbols
+++ b/tests/baselines/reference/typeFromJSInitializer.symbols
@@ -175,3 +175,22 @@ l.push('ok')
 >l : Symbol(l, Decl(a.js, 45, 3))
 >push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
 
+/** @type {(v: unknown) => v is undefined} */
+const isUndef = v => v === undefined;
+>isUndef : Symbol(isUndef, Decl(a.js, 55, 5))
+>v : Symbol(v, Decl(a.js, 55, 15))
+>v : Symbol(v, Decl(a.js, 55, 15))
+>undefined : Symbol(undefined)
+
+const e = [1, undefined];
+>e : Symbol(e, Decl(a.js, 56, 5))
+>undefined : Symbol(undefined)
+
+// should be  undefined[]
+const g = e.filter(isUndef);
+>g : Symbol(g, Decl(a.js, 59, 5))
+>e.filter : Symbol(Array.filter, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>e : Symbol(e, Decl(a.js, 56, 5))
+>filter : Symbol(Array.filter, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isUndef : Symbol(isUndef, Decl(a.js, 55, 5))
+
diff --git a/tests/baselines/reference/typeFromJSInitializer.types b/tests/baselines/reference/typeFromJSInitializer.types
index 2ed82388598..1dd46220044 100644
--- a/tests/baselines/reference/typeFromJSInitializer.types
+++ b/tests/baselines/reference/typeFromJSInitializer.types
@@ -239,3 +239,27 @@ l.push('ok')
 >push : (...items: any[]) => number
 >'ok' : "ok"
 
+/** @type {(v: unknown) => v is undefined} */
+const isUndef = v => v === undefined;
+>isUndef : (v: unknown) => v is undefined
+>v => v === undefined : (v: unknown) => v is undefined
+>v : unknown
+>v === undefined : boolean
+>v : unknown
+>undefined : undefined
+
+const e = [1, undefined];
+>e : (number | undefined)[]
+>[1, undefined] : (number | undefined)[]
+>1 : 1
+>undefined : undefined
+
+// should be  undefined[]
+const g = e.filter(isUndef);
+>g : undefined[]
+>e.filter(isUndef) : undefined[]
+>e.filter : { <S extends number | undefined>(predicate: (value: number | undefined, index: number, array: (number | undefined)[]) => value is S, thisArg?: any): S[]; (predicate: (value: number | undefined, index: number, array: (number | undefined)[]) => unknown, thisArg?: any): (number | undefined)[]; }
+>e : (number | undefined)[]
+>filter : { <S extends number | undefined>(predicate: (value: number | undefined, index: number, array: (number | undefined)[]) => value is S, thisArg?: any): S[]; (predicate: (value: number | undefined, index: number, array: (number | undefined)[]) => unknown, thisArg?: any): (number | undefined)[]; }
+>isUndef : (v: unknown) => v is undefined
+
diff --git a/tests/cases/conformance/salsa/typeFromJSInitializer.ts b/tests/cases/conformance/salsa/typeFromJSInitializer.ts
index 39d2edb39b3..fc6a08822fb 100644
--- a/tests/cases/conformance/salsa/typeFromJSInitializer.ts
+++ b/tests/cases/conformance/salsa/typeFromJSInitializer.ts
@@ -57,3 +57,10 @@ u = {}
 u = 'ok'
 
 l.push('ok')
+
+/** @type {(v: unknown) => v is undefined} */
+const isUndef = v => v === undefined;
+const e = [1, undefined];
+
+// should be  undefined[]
+const g = e.filter(isUndef);