fix corner case in unused (#5339)

fixes #5338

lib/compress.js

@@ -6943,13 +6943,16 @@ Compressor.prototype.compress = function(node) {
                     if (!drop_sym || !drop_vars || sym.id in in_use_ids) {
                         var index;
                         if (value && ((index = indexOf_assign(sym, def)) < 0 || self_assign(value.tail_node()))) {
-                            value = value.drop_side_effect_free(compressor);
-                            if (value) {
-                                AST_Node.warn("Side effects in definition of variable {name} [{file}:{line},{col}]", template(def.name));
-                                side_effects.push(value);
-                            }
                             def = def.clone();
-                            def.value = value = null;
+                            value = value.drop_side_effect_free(compressor);
+                            if (value) AST_Node.warn("Side effects in definition of variable {name} [{file}:{line},{col}]", template(def.name));
+                            if (node instanceof AST_Const) {
+                                def.value = value || make_node(AST_Number, def, { value: 0 });
+                            } else {
+                                def.value = null;
+                                if (value) side_effects.push(value);
+                            }
+                            value = null;
                             if (index >= 0) assign_in_use[sym.id][index] = def;
                         }
                         var old_def, fn;

test/compress/const.js

@@ -1843,3 +1843,16 @@ issue_5319: {
     }
     expect_stdout: true
 }
+
+issue_5338: {
+    options = {
+        unused: true,
+    }
+    input: {
+        const a = a;
+    }
+    expect: {
+        const a = a;
+    }
+    expect_stdout: true
+}

test/compress/let.js

@@ -2006,3 +2006,20 @@ issue_5319: {
     expect_stdout: "PASS"
     node_version: ">=4"
 }
+
+issue_5338: {
+    options = {
+        unused: true,
+    }
+    input: {
+        "use strict";
+        let a = a;
+    }
+    expect: {
+        "use strict";
+        a;
+        let a;
+    }
+    expect_stdout: ReferenceError("a is not defined")
+    node_version: ">=4"
+}