From deb87bb8ada5671c1c8cfeec03b3784a9abd04db Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Fri, 23 Mar 2018 09:42:43 -0700
Subject: [PATCH 1/3] Obtain constraint from destructured property when
 applicable

---
 src/compiler/checker.ts | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 29a555ee29705..65807b5347b86 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -4082,8 +4082,7 @@ namespace ts {
                     if (strictNullChecks && declaration.flags & NodeFlags.Ambient && isParameterDeclaration(declaration)) {
                         parentType = getNonNullableType(parentType);
                     }
-                    const propType = getTypeOfPropertyOfType(parentType, text);
-                    const declaredType = propType && getConstraintForLocation(propType, declaration.name);
+                    const declaredType = getConstraintForLocation(getTypeOfPropertyOfType(parentType, text), declaration.name);
                     type = declaredType && getFlowTypeOfReference(declaration, declaredType) ||
                         isNumericLiteralName(text) && getIndexTypeOfType(parentType, IndexKind.Number) ||
                         getIndexTypeOfType(parentType, IndexKind.String);
@@ -12382,7 +12381,7 @@ namespace ts {
 
         function getTypeOfDestructuredProperty(type: Type, name: PropertyName) {
             const text = getTextOfPropertyName(name);
-            return getTypeOfPropertyOfType(type, text) ||
+            return getConstraintForLocation(getTypeOfPropertyOfType(type, text), name) ||
                 isNumericLiteralName(text) && getIndexTypeOfType(type, IndexKind.Number) ||
                 getIndexTypeOfType(type, IndexKind.String) ||
                 unknownType;
@@ -13498,7 +13497,7 @@ namespace ts {
             // and the type of the node includes type variables with constraints that are nullable, we fetch the
             // apparent type of the node *before* performing control flow analysis such that narrowings apply to
             // the constraint type.
-            if (isConstraintPosition(node) && forEachType(type, typeHasNullableConstraint)) {
+            if (type && isConstraintPosition(node) && forEachType(type, typeHasNullableConstraint)) {
                 return mapType(getWidenedType(type), getBaseConstraintOrType);
             }
             return type;

From 1c2f3d228cf5634c7afc5bb6501074837358a9bb Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Fri, 23 Mar 2018 09:48:00 -0700
Subject: [PATCH 2/3] Add regression test

---
 tests/cases/compiler/destructuringWithConstraint.ts | 12 ++++++++++++
 1 file changed, 12 insertions(+)
 create mode 100644 tests/cases/compiler/destructuringWithConstraint.ts

diff --git a/tests/cases/compiler/destructuringWithConstraint.ts b/tests/cases/compiler/destructuringWithConstraint.ts
new file mode 100644
index 0000000000000..505cab86c9767
--- /dev/null
+++ b/tests/cases/compiler/destructuringWithConstraint.ts
@@ -0,0 +1,12 @@
+// @strict: true
+
+// Repro from #22823
+
+interface Props {
+    foo?: boolean;
+}
+
+function foo<P extends Props>(props: Readonly<P>) {
+    let { foo = false } = props;
+    if (foo === true) { }
+}

From ad3b3597e43c3256e78a82704bf767a5433c9a76 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Fri, 23 Mar 2018 09:48:11 -0700
Subject: [PATCH 3/3] Accept new baselines

---
 .../reference/destructuringWithConstraint.js  | 20 +++++++++++++
 .../destructuringWithConstraint.symbols       | 26 +++++++++++++++++
 .../destructuringWithConstraint.types         | 29 +++++++++++++++++++
 3 files changed, 75 insertions(+)
 create mode 100644 tests/baselines/reference/destructuringWithConstraint.js
 create mode 100644 tests/baselines/reference/destructuringWithConstraint.symbols
 create mode 100644 tests/baselines/reference/destructuringWithConstraint.types

diff --git a/tests/baselines/reference/destructuringWithConstraint.js b/tests/baselines/reference/destructuringWithConstraint.js
new file mode 100644
index 0000000000000..2d28971b67893
--- /dev/null
+++ b/tests/baselines/reference/destructuringWithConstraint.js
@@ -0,0 +1,20 @@
+//// [destructuringWithConstraint.ts]
+// Repro from #22823
+
+interface Props {
+    foo?: boolean;
+}
+
+function foo<P extends Props>(props: Readonly<P>) {
+    let { foo = false } = props;
+    if (foo === true) { }
+}
+
+
+//// [destructuringWithConstraint.js]
+"use strict";
+// Repro from #22823
+function foo(props) {
+    var _a = props.foo, foo = _a === void 0 ? false : _a;
+    if (foo === true) { }
+}
diff --git a/tests/baselines/reference/destructuringWithConstraint.symbols b/tests/baselines/reference/destructuringWithConstraint.symbols
new file mode 100644
index 0000000000000..aef7aad768e4e
--- /dev/null
+++ b/tests/baselines/reference/destructuringWithConstraint.symbols
@@ -0,0 +1,26 @@
+=== tests/cases/compiler/destructuringWithConstraint.ts ===
+// Repro from #22823
+
+interface Props {
+>Props : Symbol(Props, Decl(destructuringWithConstraint.ts, 0, 0))
+
+    foo?: boolean;
+>foo : Symbol(Props.foo, Decl(destructuringWithConstraint.ts, 2, 17))
+}
+
+function foo<P extends Props>(props: Readonly<P>) {
+>foo : Symbol(foo, Decl(destructuringWithConstraint.ts, 4, 1))
+>P : Symbol(P, Decl(destructuringWithConstraint.ts, 6, 13))
+>Props : Symbol(Props, Decl(destructuringWithConstraint.ts, 0, 0))
+>props : Symbol(props, Decl(destructuringWithConstraint.ts, 6, 30))
+>Readonly : Symbol(Readonly, Decl(lib.d.ts, --, --))
+>P : Symbol(P, Decl(destructuringWithConstraint.ts, 6, 13))
+
+    let { foo = false } = props;
+>foo : Symbol(foo, Decl(destructuringWithConstraint.ts, 7, 9))
+>props : Symbol(props, Decl(destructuringWithConstraint.ts, 6, 30))
+
+    if (foo === true) { }
+>foo : Symbol(foo, Decl(destructuringWithConstraint.ts, 7, 9))
+}
+
diff --git a/tests/baselines/reference/destructuringWithConstraint.types b/tests/baselines/reference/destructuringWithConstraint.types
new file mode 100644
index 0000000000000..a58745a6481b2
--- /dev/null
+++ b/tests/baselines/reference/destructuringWithConstraint.types
@@ -0,0 +1,29 @@
+=== tests/cases/compiler/destructuringWithConstraint.ts ===
+// Repro from #22823
+
+interface Props {
+>Props : Props
+
+    foo?: boolean;
+>foo : boolean | undefined
+}
+
+function foo<P extends Props>(props: Readonly<P>) {
+>foo : <P extends Props>(props: Readonly<P>) => void
+>P : P
+>Props : Props
+>props : Readonly<P>
+>Readonly : Readonly<T>
+>P : P
+
+    let { foo = false } = props;
+>foo : boolean
+>false : false
+>props : Readonly<P>
+
+    if (foo === true) { }
+>foo === true : boolean
+>foo : boolean
+>true : true
+}
+