From b4e83f5c12019e551f463bdf503923739d53e37d Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Sun, 25 Mar 2018 11:29:58 -0700
Subject: [PATCH] Add missing type relationship for conditional types

---
 src/compiler/checker.ts | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index e1a759fc81b..6e1bc4df603 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -10231,10 +10231,13 @@ namespace ts {
                             if (result = isRelatedTo(getTrueTypeFromConditionalType(<ConditionalType>source), getTrueTypeFromConditionalType(<ConditionalType>target), reportErrors)) {
                                 result &= isRelatedTo(getFalseTypeFromConditionalType(<ConditionalType>source), getFalseTypeFromConditionalType(<ConditionalType>target), reportErrors);
                             }
-                            if (result) {
-                                errorInfo = saveErrorInfo;
-                                return result;
-                            }
+                        }
+                        else {
+                            result = isRelatedTo(getDefaultConstraintOfConditionalType(<ConditionalType>source), getDefaultConstraintOfConditionalType(<ConditionalType>target), reportErrors);
+                        }
+                        if (result) {
+                            errorInfo = saveErrorInfo;
+                            return result;
                         }
                     }
                     else if (relation !== definitelyAssignableRelation) {