fix corner case in `unused` (#4277)

fixes #4276

2 files changed, 68 insertions, 5 deletions

diff --git a/lib/compress.js b/lib/compress.js
index 97252f7f..9eb4a9e8 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -5032,7 +5032,7 @@ merge(Compressor.prototype, {
                             }
                         }
                         var old_def, var_defs = var_defs_by_id.get(sym.id);
-                        if (!def.value) {
+                        if (!def.value && !(node instanceof AST_Let)) {
                             if (var_defs.length > 1) {
                                 AST_Node.info("Dropping declaration of variable {name} [{file}:{line},{col}]", template(def.name));
                                 remove(var_defs, def);
@@ -5071,13 +5071,18 @@ merge(Compressor.prototype, {
                                 duplicated++;
                             }
                             if (side_effects.length > 0) {
-                                if (tail.length > 0) {
-                                    side_effects.push(def.value);
-                                    def.value = make_sequence(def.value, side_effects);
-                                } else {
+                                if (tail.length == 0) {
                                     body.push(make_node(AST_SimpleStatement, node, {
                                         body: make_sequence(node, side_effects)
                                     }));
+                                } else if (def.value) {
+                                    side_effects.push(def.value);
+                                    def.value = make_sequence(def.value, side_effects);
+                                } else {
+                                    def.value = make_node(AST_UnaryPrefix, def, {
+                                        operator: "void",
+                                        expression: make_sequence(def, side_effects)
+                                    });
                                 }
                                 side_effects = [];
                             }
diff --git a/test/compress/let.js b/test/compress/let.js
index 728e40b4..20bec4d3 100644
--- a/test/compress/let.js
+++ b/test/compress/let.js
@@ -1006,3 +1006,61 @@ issue_4274_2: {
     expect_stdout: "PASS"
     node_version: ">=4"
 }
+
+issue_4276_1: {
+    options = {
+        unused: true,
+    }
+    input: {
+        "use strict";
+        try {
+            let a = b, b;
+            console.log("FAIL");
+        } catch (e) {
+            console.log("PASS");
+        }
+    }
+    expect: {
+        "use strict";
+        try {
+            let a = b, b;
+            console.log("FAIL");
+        } catch (e) {
+            console.log("PASS");
+        }
+    }
+    expect_stdout: "PASS"
+    node_version: ">=4"
+}
+
+issue_4276_2: {
+    options = {
+        unused: true,
+    }
+    input: {
+        "use strict";
+        try {
+            let a = f(), b;
+            console.log("FAIL");
+            function f() {
+                return b;
+            }
+        } catch (e) {
+            console.log("PASS");
+        }
+    }
+    expect: {
+        "use strict";
+        try {
+            let a = f(), b;
+            console.log("FAIL");
+            function f() {
+                return b;
+            }
+        } catch (e) {
+            console.log("PASS");
+        }
+    }
+    expect_stdout: "PASS"
+    node_version: ">=4"
+}