From 4b08c0582d1b672cffc8478602ba4de65ff06550 Mon Sep 17 00:00:00 2001
From: Ron Buckton <ron.buckton@microsoft.com>
Date: Wed, 6 May 2020 12:04:28 -0700
Subject: [PATCH] Fix regression in name resolution in parameter (#38351)

---
 src/compiler/checker.ts                             |  4 +++-
 .../parameterInitializersBackwardReferencing.ts     | 13 +++++++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)
 create mode 100644 tests/cases/conformance/functions/parameterInitializersBackwardReferencing.ts

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 474100799b9..529c5c1884e 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -1889,7 +1889,9 @@ namespace ts {
                             lastLocation === (location as BindingElement).name && isBindingPattern(lastLocation))) {
                             const root = getRootDeclaration(location);
                             if (root.kind === SyntaxKind.Parameter) {
-                                associatedDeclarationForContainingInitializerOrBindingName = location as BindingElement;
+                                if (!associatedDeclarationForContainingInitializerOrBindingName) {
+                                    associatedDeclarationForContainingInitializerOrBindingName = location as BindingElement;
+                                }
                             }
                         }
                         break;
diff --git a/tests/cases/conformance/functions/parameterInitializersBackwardReferencing.ts b/tests/cases/conformance/functions/parameterInitializersBackwardReferencing.ts
new file mode 100644
index 00000000000..dce4818a509
--- /dev/null
+++ b/tests/cases/conformance/functions/parameterInitializersBackwardReferencing.ts
@@ -0,0 +1,13 @@
+// @target: es5, es2015, esnext
+// @noEmit: true
+// @noTypesAndSymbols: true
+
+// https://github.com/microsoft/TypeScript/issues/38243
+
+function test0({ a = 0, b = a } = {}) {
+    return { a, b };
+}
+
+function test1({ c: { a = 0, b = a } = {} } = {}) {
+    return { a, b };
+}
\ No newline at end of file