From dd55797329ae9349fc4b8e01231e855e61c64be1 Mon Sep 17 00:00:00 2001
From: Wesley Wigham <wewigham@microsoft.com>
Date: Wed, 31 Oct 2018 12:36:46 -0700
Subject: [PATCH] Avoid this-instantiation if not necessary for relationship

---
 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 0a6cec7f6cb7e..169d4c9b64e8e 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -12044,12 +12044,15 @@ namespace ts {
                             return result;
                         }
                     }
-                    else {
-                        const instantiated = getTypeWithThisArgument(constraint, source);
-                        if (result = isRelatedTo(instantiated, target, reportErrors, /*headMessage*/ undefined, isIntersectionConstituent)) {
+                    // hi-speed no-this-instantiation check (less accurate, but avoids costly `this`-instantiation when the constraint will suffice), see #28231 for report on why this is needed
+                    else if (result = isRelatedTo(constraint, target, /*reportErrors*/ false, /*headMessage*/ undefined, isIntersectionConstituent)) {
                             errorInfo = saveErrorInfo;
                             return result;
-                        }
+                    }
+                    // slower, fuller, this-instantiated check (necessary when comparing raw `this` types from base classes), see `subclassWithPolymorphicThisIsAssignable.ts` test for example
+                    else if (result = isRelatedTo(getTypeWithThisArgument(constraint, source), target, reportErrors, /*headMessage*/ undefined, isIntersectionConstituent)) {
+                        errorInfo = saveErrorInfo;
+                        return result;
                     }
                 }
                 else if (source.flags & TypeFlags.Index) {