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] 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 29a555ee297..65807b5347b 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;