From f0ca9cfbe65efc919149e7cd74cedd186d6413ee Mon Sep 17 00:00:00 2001
From: "Alex Lam S.L" <alexlamsl@gmail.com>
Date: Mon, 16 Jan 2023 15:30:59 +0200
Subject: [PATCH] fix corner case in `collapse_vars` (#5780)

fixes #5779
---
 lib/compress.js                |  5 +++++
 test/compress/collapse_vars.js | 26 +++++++++++++++++++++++++-
 2 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/lib/compress.js b/lib/compress.js
index 4e3469ec668..98661c765f7 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -1761,6 +1761,11 @@ Compressor.prototype.compress = function(node) {
 
     var identifier_atom = makePredicate("Infinity NaN undefined");
     function is_lhs_read_only(lhs, compressor) {
+        if (lhs instanceof AST_Assign) {
+            if (lhs.operator != "=") return true;
+            if (lhs.right.tail_node().is_constant()) return true;
+            return is_lhs_read_only(lhs.left, compressor);
+        }
         if (lhs instanceof AST_Atom) return true;
         if (lhs instanceof AST_ObjectIdentity) return true;
         if (lhs instanceof AST_PropAccess) {
diff --git a/test/compress/collapse_vars.js b/test/compress/collapse_vars.js
index 13f01407f2c..45d26ea3131 100644
--- a/test/compress/collapse_vars.js
+++ b/test/compress/collapse_vars.js
@@ -9864,7 +9864,8 @@ issue_5276: {
     }
     expect: {
         var a = A = "PASS";
-        a.p = a.p + null - 42;
+        a.p += null;
+        a.p -= 42;
         console.log(a);
     }
     expect_stdout: "PASS"
@@ -10148,3 +10149,26 @@ issue_5719: {
     }
     expect_stdout: "PASS"
 }
+
+issue_5779: {
+    options = {
+        collapse_vars: true,
+        evaluate: true,
+        pure_getters: "strict",
+        reduce_vars: true,
+        toplevel: true,
+    }
+    input: {
+        var a = A = "foo";
+        a.p = 42;
+        if (a && !a.p)
+            console.log("PASS");
+    }
+    expect: {
+        var a = A = "foo";
+        a.p = 42;
+        if (a, !a.p)
+            console.log("PASS");
+    }
+    expect_stdout: "PASS"
+}