From 595cb11f22d4bb3fe4bbf3e4d0366c6b5903c57a Mon Sep 17 00:00:00 2001
From: Nathan Shively-Sanders <nathansa@microsoft.com>
Date: Thu, 8 Jun 2017 11:01:32 -0700
Subject: [PATCH] Excess property checks for discriminated unions

This uses the same code as #14006, which improves error messages for
discriminated unions.
---
 src/compiler/checker.ts | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 266fbe573be..ddfebd84ad6 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -8964,6 +8964,13 @@ namespace ts {
                         (isTypeSubsetOf(globalObjectType, target) || (!isComparingJsxAttributes && isEmptyObjectType(target)))) {
                         return false;
                     }
+                    if (target.flags & TypeFlags.Union) {
+                        const discriminantType = findMatchingDiscriminantType(source, target as UnionType);
+                        if (discriminantType)  {
+                            // check excess properties against discriminant type only, not the entire union
+                            return hasExcessProperties(source, discriminantType, reportErrors);
+                        }
+                    }
                     for (const prop of getPropertiesOfObjectType(source)) {
                         if (!isKnownProperty(target, prop.name, isComparingJsxAttributes)) {
                             if (reportErrors) {