From 938368ba219f29703b9f9e67c6fb0d1c0723220a Mon Sep 17 00:00:00 2001
From: "Alex Lam S.L" <alexlamsl@gmail.com>
Date: Thu, 14 May 2020 00:52:42 +0100
Subject: [PATCH] enhance `collapse_vars` (#3896)

---
 lib/compress.js                | 10 ++++------
 test/compress/collapse_vars.js | 35 ++++++++++++++++++++++++++++++++++
 2 files changed, 39 insertions(+), 6 deletions(-)

diff --git a/lib/compress.js b/lib/compress.js
index 8cf7262a..93dbe063 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -1350,7 +1350,7 @@ merge(Compressor.prototype, {
                     var lhs = get_lhs(candidate);
                     var side_effects = lhs && lhs.has_side_effects(compressor);
                     var scan_lhs = lhs && !side_effects && !is_lhs_read_only(lhs, compressor);
-                    var scan_rhs = foldable(get_rhs(candidate));
+                    var scan_rhs = foldable(candidate);
                     if (!scan_lhs && !scan_rhs) continue;
                     var modify_toplevel = false;
                     // Locate symbols which may execute code outside of scanning range
@@ -1841,10 +1841,6 @@ merge(Compressor.prototype, {
                 }
             }
 
-            function get_rhs(expr) {
-                return candidate instanceof AST_Assign && candidate.operator == "=" && candidate.right;
-            }
-
             function invariant(expr) {
                 if (expr instanceof AST_Array) return false;
                 if (expr instanceof AST_Binary && lazy_op[expr.operator]) {
@@ -1859,7 +1855,9 @@ merge(Compressor.prototype, {
             }
 
             function foldable(expr) {
-                if (!expr) return false;
+                while (expr instanceof AST_Assign && expr.operator == "=") {
+                    expr = expr.right;
+                }
                 if (expr instanceof AST_SymbolRef) {
                     var value = expr.evaluate(compressor);
                     if (value === expr) return rhs_exact_match;
diff --git a/test/compress/collapse_vars.js b/test/compress/collapse_vars.js
index dee370cf..0dd8754d 100644
--- a/test/compress/collapse_vars.js
+++ b/test/compress/collapse_vars.js
@@ -8015,3 +8015,38 @@ issue_3891: {
     }
     expect_stdout: "function"
 }
+
+issue_3894: {
+    options = {
+        collapse_vars: true,
+        inline: true,
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        function log(msg) {
+            console.log(msg ? "FAIL" : "PASS");
+        }
+        var a, c;
+        (function(b) {
+            a = c = 0,
+            log(b);
+        })(-0);
+        log(a);
+        log(c);
+    }
+    expect: {
+        function log(msg) {
+            console.log(msg ? "FAIL" : "PASS");
+        }
+        var a, c;
+        void log(-(a = c = 0));
+        log(a);
+        log(c);
+    }
+    expect_stdout: [
+        "PASS",
+        "PASS",
+        "PASS",
+    ]
+}
-- 
2.34.1