fix corner case in `dead_code` (#4984)

2 files changed, 35 insertions, 3 deletions

diff --git a/lib/compress.js b/lib/compress.js
index 7ea1e533..7ea93ecd 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -11085,9 +11085,7 @@ merge(Compressor.prototype, {
                         ]).optimize(compressor);
                     }
                 }
-            } else if (self.left instanceof AST_SymbolRef && all(self.left.definition().orig, function(sym) {
-                return !(sym instanceof AST_SymbolConst);
-            })) {
+            } else if (self.left instanceof AST_SymbolRef && can_drop_symbol(self.left, compressor)) {
                 var parent;
                 if (self.operator == "=" && self.left.equivalent_to(self.right)
                     && !((parent = compressor.parent()) instanceof AST_UnaryPrefix && parent.operator == "delete")) {
diff --git a/test/compress/let.js b/test/compress/let.js
index c59308c4..4df5bb01 100644
--- a/test/compress/let.js
+++ b/test/compress/let.js
@@ -20,6 +20,39 @@ retain_block: {
     node_version: ">=4"
 }
 
+retain_assignment: {
+    options = {
+        dead_code: true,
+        reduce_vars: true,
+    }
+    input: {
+        "use strict";
+        function f() {
+            return a = 0;
+            let a;
+        }
+        try {
+            f();
+        } catch (e) {
+            console.log("PASS");
+        }
+    }
+    expect: {
+        "use strict";
+        function f() {
+            return a = 0;
+            let a;
+        }
+        try {
+            f();
+        } catch (e) {
+            console.log("PASS");
+        }
+    }
+    expect_stdout: "PASS"
+    node_version: ">=4"
+}
+
 retain_catch: {
     options = {
         dead_code: true,
@@ -897,6 +930,7 @@ issue_4210: {
 issue_4212_1: {
     options = {
         dead_code: true,
+        reduce_vars: true,
     }
     input: {
         "use strict";