From 5c889299f4ddb4ba53b704a423da01e2d21275b9 Mon Sep 17 00:00:00 2001
From: Nathan Shively-Sanders <293473+sandersn@users.noreply.github.com>
Date: Wed, 17 Jan 2018 13:21:10 -0800
Subject: [PATCH] Indexed access relation check object+index types

Previously, it only check the object types, and only if the index types
were identical. Now both checks call `isRelatedTo` recursively.
---
 src/compiler/checker.ts | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 489fff16e2c..0cfb6d1a381 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -9632,7 +9632,7 @@ namespace ts {
                 }
                 else if (target.flags & TypeFlags.IndexedAccess) {
                     // A type S is related to a type T[K] if S is related to A[K], where K is string-like and
-                    // A is the apparent type of T.
+                    // A is the constraint of T.
                     const constraint = getConstraintOfIndexedAccess(<IndexedAccessType>target);
                     if (constraint) {
                         if (result = isRelatedTo(source, constraint, reportErrors)) {
@@ -9668,7 +9668,7 @@ namespace ts {
                 }
                 else if (source.flags & TypeFlags.IndexedAccess) {
                     // A type S[K] is related to a type T if A[K] is related to T, where K is string-like and
-                    // A is the apparent type of S.
+                    // A is the constraint of S.
                     const constraint = getConstraintOfIndexedAccess(<IndexedAccessType>source);
                     if (constraint) {
                         if (result = isRelatedTo(constraint, target, reportErrors)) {
@@ -9676,10 +9676,11 @@ namespace ts {
                             return result;
                         }
                     }
-                    else if (target.flags & TypeFlags.IndexedAccess && (<IndexedAccessType>source).indexType === (<IndexedAccessType>target).indexType) {
-                        // if we have indexed access types with identical index types, see if relationship holds for
-                        // the two object types.
+                    else if (target.flags & TypeFlags.IndexedAccess) {
                         if (result = isRelatedTo((<IndexedAccessType>source).objectType, (<IndexedAccessType>target).objectType, reportErrors)) {
+                            result &= isRelatedTo((<IndexedAccessType>source).indexType, (<IndexedAccessType>target).indexType, reportErrors);
+                        }
+                        if (result) {
                             errorInfo = saveErrorInfo;
                             return result;
                         }