From 38fd166612877f532401d61e9b74adbf158fa6ba Mon Sep 17 00:00:00 2001
From: alexlamsl <alexlamsl@gmail.com>
Date: Wed, 13 Dec 2017 01:17:46 +0800
Subject: [PATCH] improve `collapse_vars` on side-effect-free replacements

---
 lib/compress.js                |  5 +++--
 test/compress/collapse_vars.js | 19 +++++++++++++++++++
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/lib/compress.js b/lib/compress.js
index 14e83617..286a6b7e 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -895,10 +895,11 @@ merge(Compressor.prototype, {
                     || node instanceof AST_Call && lhs instanceof AST_PropAccess && lhs.equivalent_to(node.expression)
                     || node instanceof AST_Debugger
                     || node instanceof AST_IterationStatement && !(node instanceof AST_For)
-                    || node instanceof AST_SymbolRef && !node.is_declared(compressor)
                     || node instanceof AST_Try
                     || node instanceof AST_With
-                    || parent instanceof AST_For && node !== parent.init) {
+                    || parent instanceof AST_For && node !== parent.init
+                    || (side_effects || !replace_all)
+                        && (node instanceof AST_SymbolRef && !node.is_declared(compressor))) {
                     abort = true;
                     return node;
                 }
diff --git a/test/compress/collapse_vars.js b/test/compress/collapse_vars.js
index 330667dd..4cdf1a4e 100644
--- a/test/compress/collapse_vars.js
+++ b/test/compress/collapse_vars.js
@@ -3805,3 +3805,22 @@ may_throw: {
         }
     }
 }
+
+side_effect_free_replacement: {
+    options = {
+        collapse_vars: true,
+        inline: true,
+        side_effects: true,
+        unused: true,
+    }
+    input: {
+        var b;
+        (function(a) {
+            x(a);
+        })(b);
+    }
+    expect: {
+        var b;
+        x(b);
+    }
+}