From 23d74bedeb91742cd8bc090cb96812ef99dda0c5 Mon Sep 17 00:00:00 2001
From: "Alex Lam S.L" <alexlamsl@gmail.com>
Date: Mon, 17 Jun 2024 09:11:11 +0300
Subject: [PATCH] fix corner cases in `join_vars` (#5852)

fixes #5849
fixes #5850
---
 lib/compress.js            | 18 +++++++++---------
 test/compress/join_vars.js | 35 +++++++++++++++++++++++++++--------
 test/compress/spreads.js   | 21 +++++++++++++++++++++
 3 files changed, 57 insertions(+), 17 deletions(-)

diff --git a/lib/compress.js b/lib/compress.js
index 26dc1505dfd..ce5c898277f 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -4300,18 +4300,18 @@ Compressor.prototype.compress = function(node) {
                 if (prop instanceof AST_Node) return;
                 if (!RE_POSITIVE_INTEGER.test("" + prop)) return;
                 prop = +prop;
-                var len = value.elements.length;
+                var elements = value.elements;
+                var len = elements.length;
                 if (prop > len + 4) return;
+                for (var i = Math.min(len, prop + 1); --i >= 0;) {
+                    if (elements[i] instanceof AST_Spread) return;
+                }
                 if (prop < len) {
-                    var element = value.elements[prop];
-                    if (element instanceof AST_Hole) {
-                        value.elements[prop] = node.right;
-                    } else {
-                        value.elements[prop] = make_sequence(node, [ element, node.right ]).optimize(compressor);
-                    }
+                    var element = elements[prop].drop_side_effect_free(compressor);
+                    elements[prop] = element ? make_sequence(node, [ element, node.right ]) : node.right;
                 } else {
-                    while (prop > len) value.elements[len++] = make_node(AST_Hole, value);
-                    value.elements[prop] = node.right;
+                    while (prop > len) elements[len++] = make_node(AST_Hole, value);
+                    elements[prop] = node.right;
                 }
                 return true;
             }
diff --git a/test/compress/join_vars.js b/test/compress/join_vars.js
index 80c89f330cc..cbf8137a8a1 100644
--- a/test/compress/join_vars.js
+++ b/test/compress/join_vars.js
@@ -23,7 +23,7 @@ join_array_assignments_1: {
     }
     input: {
         console.log(function () {
-            var a = ["foo", , "bar"];
+            var a = [ "foo", , "bar" ];
             a[1] = "baz";
             a[7] = "moo";
             a[0] = "moz";
@@ -32,7 +32,7 @@ join_array_assignments_1: {
     }
     expect: {
         console.log(function () {
-            var a = [("foo", "moz"), "baz", "bar", , , , , "moo"];
+            var a = [ "moz", "baz", "bar", , , , , "moo" ];
             return a;
         }().join());
     }
@@ -46,7 +46,7 @@ join_array_assignments_2: {
     }
     input: {
         console.log(function () {
-            var a = ["foo"];
+            var a = [ "foo" ];
             a[1] = "bar";
             a[7] = "baz";
             a[2] = "moo";
@@ -55,7 +55,7 @@ join_array_assignments_2: {
     }
     expect: {
         console.log(function () {
-            var a = ["foo", "bar"];
+            var a = [ "foo", "bar" ];
             a[7] = "baz";
             a[2] = "moo";
             return a;
@@ -71,7 +71,7 @@ join_array_assignments_3: {
     }
     input: {
         console.log(function () {
-            var a = ["foo"];
+            var a = [ "foo" ];
             a[1] = "bar";
             a.b = "baz";
             a[2] = "moo";
@@ -80,7 +80,7 @@ join_array_assignments_3: {
     }
     expect: {
         console.log(function () {
-            var a = ["foo", "bar"];
+            var a = [ "foo", "bar" ];
             a.b = "baz";
             a[2] = "moo";
             return a;
@@ -97,7 +97,7 @@ join_array_assignments_4: {
     }
     input: {
         console.log(function () {
-            var a = ["foo"];
+            var a = [ "foo" ];
             a[0] = "bar";
             a[1] = a;
             a[2] = "baz";
@@ -106,7 +106,7 @@ join_array_assignments_4: {
     }
     expect: {
         console.log(function () {
-            var a = ["bar"];
+            var a = [ "bar" ];
             a[1] = a;
             a[2] = "baz";
             return a;
@@ -1508,3 +1508,22 @@ issue_5831: {
     }
     expect_stdout: "PASS"
 }
+
+issue_5849: {
+    options = {
+        evaluate: true,
+        join_vars: true,
+        side_effects: true,
+    }
+    input: {
+        var a;
+        a = [ 42 ];
+        a[0] = "PASS";
+        console.log(a.join(""));
+    }
+    expect: {
+        var a, a = [ "PASS" ];
+        console.log(a.join(""));
+    }
+    expect_stdout: "PASS"
+}
diff --git a/test/compress/spreads.js b/test/compress/spreads.js
index 82f69120e61..9ab23b4adcc 100644
--- a/test/compress/spreads.js
+++ b/test/compress/spreads.js
@@ -1253,3 +1253,24 @@ issue_5602: {
     ]
     node_version: ">=6"
 }
+
+issue_5850: {
+    options = {
+        evaluate: true,
+        join_vars: true,
+        unused: true,
+    }
+    input: {
+        var a = [ ..."FAIL" ];
+        a[0] = "P";
+        a[2] = a[3] = "S";
+        console.log(a.join(""));
+    }
+    expect: {
+        var a = [ ..."FAIL" ];
+        a[0] = "P";
+        a[2] = a[3] = "S";
+        console.log(a.join(""));
+    }
+    node_version: ">=6"
+}