From 7cdd58c95065378b83b1bf5a43728bfd8eb8bd2d Mon Sep 17 00:00:00 2001
From: Mihail Stoykov <M.Stoikov@gmail.com>
Date: Mon, 23 Jun 2025 18:50:18 +0300
Subject: [PATCH] bugfix: this being wrong when rest arguments are used

During a previous fix it seems that rest parameters weren't taken into
consideration and currently `this` refers to them in the provided test.

The test in question is a very simplified variation of a real code.

It seems all of the following things are relevant for the bug to occur:
1. method with only a rest parameter
2. usage of `this`
3. usage of the parameter
4. All of the above within a lambda inside the method
---
 compiler_expr.go |  2 +-
 compiler_test.go | 23 +++++++++++++++++++++++
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/compiler_expr.go b/compiler_expr.go
index 71fdad38..a12a335c 100644
--- a/compiler_expr.go
+++ b/compiler_expr.go
@@ -1701,7 +1701,7 @@ func (e *compiledFunctionLiteral) compile() (prg *Program, name unistring.String
 
 	stashSize, stackSize := s.finaliseVarAlloc(0)
 
-	if needInitThis && (s.numArgs > 0 && !s.argsInStash || stackSize > 0) {
+	if needInitThis && ((s.numArgs > 0 || hasPatterns) && !s.argsInStash || stackSize > 0) {
 		code[preambleLen-delta] = initStashP(code[preambleLen-delta].(initStash))
 		delta++
 		code[preambleLen-delta] = loadStack(0)
diff --git a/compiler_test.go b/compiler_test.go
index 97011fc0..7581de99 100644
--- a/compiler_test.go
+++ b/compiler_test.go
@@ -5965,6 +5965,29 @@ func TestThisInStashCtor(t *testing.T) {
 	testScript(SCRIPT, _undefined, t)
 }
 
+func TestWrongThis(t *testing.T) {
+	const SCRIPT = `
+	class mine {
+		constructor (arg) {
+			this.field = arg
+		}
+
+		f(...argument) {
+			if (this.field != "something") {
+				throw "wrong " + this.field + " Object.keys:" + Object.keys(this)
+			}
+			let s = () => {
+				for (const arg of argument) arg.call()
+				return this.field
+			}
+		}
+	}
+	const a = new mine("something")
+	a.f({"SomeKey": "here"})
+`
+	testScript(SCRIPT, _undefined, t)
+}
+
 /*
 func TestBabel(t *testing.T) {
 	src, err := os.ReadFile("babel7.js")