From 4b813e310c3e5e7b758b85eada14ba45b898233e Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Wed, 10 Apr 2019 17:05:07 -1000
Subject: [PATCH] Simplify non-inferrable property check to rely on propagation

---
 src/compiler/checker.ts | 10 ++--------
 1 file changed, 2 insertions(+), 8 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index a1e21e010fb..e3fda9e6e42 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -14502,16 +14502,10 @@ namespace ts {
         }
 
         function createReverseMappedType(source: Type, target: MappedType, constraint: IndexType) {
-            const properties = getPropertiesOfType(source);
-            if (properties.length === 0 && !getIndexInfoOfType(source, IndexKind.String)) {
-                return undefined;
-            }
             // If any property contains context sensitive functions that have been skipped, the source type
             // is incomplete and we can't infer a meaningful input type.
-            for (const prop of properties) {
-                if (getObjectFlags(getTypeOfSymbol(prop)) & ObjectFlags.NonInferrableType) {
-                    return undefined;
-                }
+            if (getObjectFlags(source) & ObjectFlags.NonInferrableType || getPropertiesOfType(source).length === 0 && !getIndexInfoOfType(source, IndexKind.String)) {
+                return undefined;
             }
             // For arrays and tuples we infer new arrays and tuples where the reverse mapping has been
             // applied to the element type(s).