From f248567dabd1b9cc11bb53d815e79c80becaee51 Mon Sep 17 00:00:00 2001
From: Nathan Shively-Sanders <293473+sandersn@users.noreply.github.com>
Date: Wed, 22 Apr 2020 09:56:32 -0700
Subject: [PATCH] Filter undefined only in binding patterns in params (#38116)

initialiser. But this is only correct when the initialiser is for a
parameter. For example:

```ts
declare let x: { s: string } | undefined;
const { s } = x;
```

This PR removes undefined from the type of a binding pattern only when
the binding pattern's parent is a parameter. This fixes the regression
from 3.8. However, it's still not the ideal fix; we should be able to
use control flow to solve this problem. Consider:

```ts
const { s }: { s: string } | undefined = { s: 'hi' }
declare function f({ s }: { s: string } | undefined = { s: 'hi' }): void
```

Neither line should have an error, but the first does in 3.8 and after
this change.
---
 src/compiler/checker.ts                            |  2 +-
 ...rInitalizedVariablesFiltersUndefined.errors.txt | 14 ++++++++++++++
 ...alTypeForInitalizedVariablesFiltersUndefined.js |  7 ++++++-
 ...eForInitalizedVariablesFiltersUndefined.symbols |  8 ++++++++
 ...ypeForInitalizedVariablesFiltersUndefined.types |  8 ++++++++
 ...alTypeForInitalizedVariablesFiltersUndefined.ts |  5 ++++-
 6 files changed, 41 insertions(+), 3 deletions(-)
 create mode 100644 tests/baselines/reference/contextualTypeForInitalizedVariablesFiltersUndefined.errors.txt

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 89be1e0aa7..9014e2f43f 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -7306,7 +7306,7 @@ namespace ts {
                 parentType = getNonNullableType(parentType);
             }
             // Filter `undefined` from the type we check against if the parent has an initializer (which handles the `undefined` case implicitly)
-            else if (strictNullChecks && pattern.parent.initializer) {
+            else if (strictNullChecks && pattern.parent.initializer && isParameter(pattern.parent)) {
                 parentType = getTypeWithFacts(parentType, TypeFacts.NEUndefined);
             }
 
diff --git a/tests/baselines/reference/contextualTypeForInitalizedVariablesFiltersUndefined.errors.txt b/tests/baselines/reference/contextualTypeForInitalizedVariablesFiltersUndefined.errors.txt
new file mode 100644
index 0000000000..32ff831d2b
--- /dev/null
+++ b/tests/baselines/reference/contextualTypeForInitalizedVariablesFiltersUndefined.errors.txt
@@ -0,0 +1,14 @@
+tests/cases/compiler/contextualTypeForInitalizedVariablesFiltersUndefined.ts(7,9): error TS2339: Property 's' does not exist on type '{ s: string; } | undefined'.
+
+
+==== tests/cases/compiler/contextualTypeForInitalizedVariablesFiltersUndefined.ts (1 errors) ====
+    const fInferred = ({ a = 0 } = {}) => a;
+    // const fInferred: ({ a }?: { a?: number; }) => number
+    
+    const fAnnotated: typeof fInferred = ({ a = 0 } = {}) => a;
+    
+    declare var t: { s: string } | undefined;
+    const { s } = t;
+            ~
+!!! error TS2339: Property 's' does not exist on type '{ s: string; } | undefined'.
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/contextualTypeForInitalizedVariablesFiltersUndefined.js b/tests/baselines/reference/contextualTypeForInitalizedVariablesFiltersUndefined.js
index 9c2a6eaa78..8311b6ff48 100644
--- a/tests/baselines/reference/contextualTypeForInitalizedVariablesFiltersUndefined.js
+++ b/tests/baselines/reference/contextualTypeForInitalizedVariablesFiltersUndefined.js
@@ -2,7 +2,11 @@
 const fInferred = ({ a = 0 } = {}) => a;
 // const fInferred: ({ a }?: { a?: number; }) => number
 
-const fAnnotated: typeof fInferred = ({ a = 0 } = {}) => a;
+const fAnnotated: typeof fInferred = ({ a = 0 } = {}) => a;
+
+declare var t: { s: string } | undefined;
+const { s } = t;
+
 
 //// [contextualTypeForInitalizedVariablesFiltersUndefined.js]
 "use strict";
@@ -15,3 +19,4 @@ var fAnnotated = function (_a) {
     var _b = (_a === void 0 ? {} : _a).a, a = _b === void 0 ? 0 : _b;
     return a;
 };
+var s = t.s;
diff --git a/tests/baselines/reference/contextualTypeForInitalizedVariablesFiltersUndefined.symbols b/tests/baselines/reference/contextualTypeForInitalizedVariablesFiltersUndefined.symbols
index 7b67571901..81ae53f941 100644
--- a/tests/baselines/reference/contextualTypeForInitalizedVariablesFiltersUndefined.symbols
+++ b/tests/baselines/reference/contextualTypeForInitalizedVariablesFiltersUndefined.symbols
@@ -12,3 +12,11 @@ const fAnnotated: typeof fInferred = ({ a = 0 } = {}) => a;
 >a : Symbol(a, Decl(contextualTypeForInitalizedVariablesFiltersUndefined.ts, 3, 39))
 >a : Symbol(a, Decl(contextualTypeForInitalizedVariablesFiltersUndefined.ts, 3, 39))
 
+declare var t: { s: string } | undefined;
+>t : Symbol(t, Decl(contextualTypeForInitalizedVariablesFiltersUndefined.ts, 5, 11))
+>s : Symbol(s, Decl(contextualTypeForInitalizedVariablesFiltersUndefined.ts, 5, 16))
+
+const { s } = t;
+>s : Symbol(s, Decl(contextualTypeForInitalizedVariablesFiltersUndefined.ts, 6, 7))
+>t : Symbol(t, Decl(contextualTypeForInitalizedVariablesFiltersUndefined.ts, 5, 11))
+
diff --git a/tests/baselines/reference/contextualTypeForInitalizedVariablesFiltersUndefined.types b/tests/baselines/reference/contextualTypeForInitalizedVariablesFiltersUndefined.types
index d8278124ff..58580fdacc 100644
--- a/tests/baselines/reference/contextualTypeForInitalizedVariablesFiltersUndefined.types
+++ b/tests/baselines/reference/contextualTypeForInitalizedVariablesFiltersUndefined.types
@@ -18,3 +18,11 @@ const fAnnotated: typeof fInferred = ({ a = 0 } = {}) => a;
 >{} : {}
 >a : number
 
+declare var t: { s: string } | undefined;
+>t : { s: string; } | undefined
+>s : string
+
+const { s } = t;
+>s : any
+>t : { s: string; } | undefined
+
diff --git a/tests/cases/compiler/contextualTypeForInitalizedVariablesFiltersUndefined.ts b/tests/cases/compiler/contextualTypeForInitalizedVariablesFiltersUndefined.ts
index 268544f819..cfa8d8d682 100644
--- a/tests/cases/compiler/contextualTypeForInitalizedVariablesFiltersUndefined.ts
+++ b/tests/cases/compiler/contextualTypeForInitalizedVariablesFiltersUndefined.ts
@@ -2,4 +2,7 @@
 const fInferred = ({ a = 0 } = {}) => a;
 // const fInferred: ({ a }?: { a?: number; }) => number
 
-const fAnnotated: typeof fInferred = ({ a = 0 } = {}) => a;
\ No newline at end of file
+const fAnnotated: typeof fInferred = ({ a = 0 } = {}) => a;
+
+declare var t: { s: string } | undefined;
+const { s } = t;