From 7181c2af0922bf5d4f343abb5f9d2977487a5989 Mon Sep 17 00:00:00 2001
From: Wesley Wigham <wewigham@microsoft.com>
Date: Fri, 1 Nov 2019 15:17:40 -0700
Subject: [PATCH] Limit simplification to single prop or partial types

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

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 5959ba3c3d520..e9e6877a0dddf 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -12526,6 +12526,12 @@ namespace ts {
             return isEmptyObjectType(type) || !!(type.flags & (TypeFlags.Null | TypeFlags.Undefined | TypeFlags.BooleanLike | TypeFlags.NumberLike | TypeFlags.BigIntLike | TypeFlags.StringLike | TypeFlags.EnumLike | TypeFlags.NonPrimitive | TypeFlags.Index));
         }
 
+        function isSinglePropertyAnonymousObjectType(type: Type) {
+            return !!(type.flags & TypeFlags.Object) &&
+                !!(getObjectFlags(type) & ObjectFlags.Anonymous) &&
+                (length(getPropertiesOfType(type)) === 1 || every(getPropertiesOfType(type), p => !!(p.flags & SymbolFlags.Optional)));
+        }
+
         function tryMergeUnionOfObjectTypeAndEmptyObject(type: UnionType, readonly: boolean): Type | undefined {
             if (type.types.length === 2) {
                 const firstType = type.types[0];
@@ -12533,10 +12539,10 @@ namespace ts {
                 if (every(type.types, isEmptyObjectTypeOrSpreadsIntoEmptyObject)) {
                     return isEmptyObjectType(firstType) ? firstType : isEmptyObjectType(secondType) ? secondType : emptyObjectType;
                 }
-                if (isEmptyObjectTypeOrSpreadsIntoEmptyObject(firstType)) {
+                if (isEmptyObjectTypeOrSpreadsIntoEmptyObject(firstType) && isSinglePropertyAnonymousObjectType(secondType)) {
                     return getAnonymousPartialType(secondType);
                 }
-                if (isEmptyObjectTypeOrSpreadsIntoEmptyObject(secondType)) {
+                if (isEmptyObjectTypeOrSpreadsIntoEmptyObject(secondType) && isSinglePropertyAnonymousObjectType(firstType)) {
                     return getAnonymousPartialType(firstType);
                 }
             }