From bbca9de9cd7e715bf62a8c624cd3f53e4386b566 Mon Sep 17 00:00:00 2001
From: "Alex Lam S.L" <alexlamsl@gmail.com>
Date: Fri, 7 May 2021 20:58:29 +0100
Subject: [PATCH] fix corner case in `collapse_vars` (#4919)

fixes #4918
---
 lib/compress.js                | 44 ++++++++++++++++------------------
 test/compress/collapse_vars.js | 23 ++++++++++++++++++
 2 files changed, 44 insertions(+), 23 deletions(-)

diff --git a/lib/compress.js b/lib/compress.js
index fdf46fb9ae..af9ff7e30b 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -2546,12 +2546,7 @@ merge(Compressor.prototype, {
                 if (parent instanceof AST_PropAccess) {
                     var exp = parent.expression;
                     if (exp === node) return find_stop_unused(parent, level + 1);
-                    var sym = root_expr(exp);
-                    if (!(sym instanceof AST_SymbolRef)) return find_stop_unused(parent, level + 1);
-                    var lvalue = lvalues.get(sym.name);
-                    return !lvalue || all(lvalue, function(lhs) {
-                        return !(lhs instanceof AST_PropAccess);
-                    }) ? find_stop_unused(parent, level + 1) : node;
+                    return check_expr(exp);
                 }
                 if (parent instanceof AST_Sequence) return find_stop_unused(parent, level + 1);
                 if (parent instanceof AST_SimpleStatement) return find_stop_unused(parent, level + 1);
@@ -2562,25 +2557,28 @@ merge(Compressor.prototype, {
                 if (parent instanceof AST_Yield) return node;
                 return null;
 
+                function check_expr(expr) {
+                    var replace = can_replace;
+                    can_replace = false;
+                    var after = stop_after;
+                    var if_hit = stop_if_hit;
+                    var stack = scanner.stack;
+                    scanner.stack = [ parent ];
+                    expr.transform(scanner);
+                    scanner.stack = stack;
+                    stop_if_hit = if_hit;
+                    stop_after = after;
+                    can_replace = replace;
+                    if (abort) {
+                        abort = false;
+                        return node;
+                    }
+                    return find_stop_unused(parent, level + 1);
+                }
+
                 function check_assignment(lhs) {
                     if (may_throw(parent)) return node;
-                    if (lhs !== node && lhs instanceof AST_Destructured) {
-                        var replace = can_replace;
-                        can_replace = false;
-                        var after = stop_after;
-                        var if_hit = stop_if_hit;
-                        var stack = scanner.stack;
-                        scanner.stack = [ parent ];
-                        lhs.transform(scanner);
-                        scanner.stack = stack;
-                        stop_if_hit = if_hit;
-                        stop_after = after;
-                        can_replace = replace;
-                        if (abort) {
-                            abort = false;
-                            return node;
-                        }
-                    }
+                    if (lhs !== node && lhs instanceof AST_Destructured) return check_expr(lhs);
                     return find_stop_unused(parent, level + 1);
                 }
             }
diff --git a/test/compress/collapse_vars.js b/test/compress/collapse_vars.js
index 8500670352..e0c139bd4c 100644
--- a/test/compress/collapse_vars.js
+++ b/test/compress/collapse_vars.js
@@ -9171,3 +9171,26 @@ issue_4914: {
     }
     expect_stdout: "function"
 }
+
+issue_4918: {
+    options = {
+        collapse_vars: true,
+    }
+    input: {
+        var a = "FAIL";
+        ({
+            get 42() {
+                console.log(a);
+            }
+        }[a = "PASS", 42] += "PASS");
+    }
+    expect: {
+        var a = "FAIL";
+        ({
+            get 42() {
+                console.log(a);
+            }
+        }[a = "PASS", 42] += "PASS");
+    }
+    expect_stdout: "PASS"
+}