From 8bbbc5178ebee10eb9fa69a706b29f40cc2aa3b9 Mon Sep 17 00:00:00 2001
From: "Alex Lam S.L." <alexlamsl@gmail.com>
Date: Sun, 4 Aug 2024 16:41:46 +0300
Subject: [PATCH] fix corner case in `unused` (#5906)

fixes #5905
---
 lib/compress.js                  | 17 +++++++++--------
 test/compress/optional-chains.js | 24 ++++++++++++++++++++++++
 2 files changed, 33 insertions(+), 8 deletions(-)

diff --git a/lib/compress.js b/lib/compress.js
index 22a3001ff3..b515c4259c 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -9349,17 +9349,18 @@ Compressor.prototype.compress = function(node) {
             var expr = self.expression;
             if (expr.may_throw_on_access(compressor)) return self;
             var prop = self.property;
-            var optional = self.optional;
-            if (!optional) {
-                expr = expr.drop_side_effect_free(compressor, first_in_statement);
-                if (!expr) return prop.drop_side_effect_free(compressor, first_in_statement);
+            if (self.optional) {
+                prop = prop.drop_side_effect_free(compressor);
+                if (!prop) return expr.drop_side_effect_free(compressor, first_in_statement);
+                self = self.clone();
+                self.property = prop;
+                return self;
             }
+            expr = expr.drop_side_effect_free(compressor, first_in_statement);
+            if (!expr) return prop.drop_side_effect_free(compressor, first_in_statement);
             prop = prop.drop_side_effect_free(compressor);
             if (!prop) return expr;
-            if (!optional) return make_sequence(self, [ expr, prop ]);
-            self = self.clone();
-            self.property = prop;
-            return self;
+            return make_sequence(self, [ expr, prop ]);
         });
         def(AST_SymbolRef, function(compressor) {
             return this.is_declared(compressor) && can_drop_symbol(this, compressor) ? null : this;
diff --git a/test/compress/optional-chains.js b/test/compress/optional-chains.js
index cd12b7bda2..a1594cf618 100644
--- a/test/compress/optional-chains.js
+++ b/test/compress/optional-chains.js
@@ -669,3 +669,27 @@ issue_5900: {
     expect_stdout: "PASS"
     node_version: ">=14"
 }
+
+issue_5905: {
+    options = {
+        pure_getters: "strict",
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a;
+        do {
+            var b = a++;
+            var c = c ?? b?.[42];
+        } while (console.log("PASS"));
+    }
+    expect: {
+        var a;
+        do {
+            a++;
+        } while (console.log("PASS"));
+    }
+    expect_stdout: "PASS"
+    node_version: ">=14"
+}