From 5bb6487bef82e034144c1eba2cbe7be3af3ef7f2 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Fri, 17 Apr 2020 10:33:28 -0700
Subject: [PATCH] Check for this.xxx access to previously declared
 uninitialized property

---
 src/compiler/checker.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index e6f8f62fe31..47300797cb9 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -1366,7 +1366,7 @@ namespace ts {
                 return sourceFiles.indexOf(declarationFile) <= sourceFiles.indexOf(useFile);
             }
 
-            if (declaration.pos <= usage.pos) {
+            if (declaration.pos <= usage.pos && !(isPropertyDeclaration(declaration) && isThisProperty(usage.parent) && !declaration.initializer && !declaration.exclamationToken)) {
                 // declaration is before usage
                 if (declaration.kind === SyntaxKind.BindingElement) {
                     // still might be illegal if declaration and usage are both binding elements (eg var [a = b, b = b] = [1, 2])