fix corner case in unused (#5086)

fixes #5085

Author: alexlamsl

lib/compress.js

@@ -6513,8 +6513,10 @@ merge(Compressor.prototype, {
                             return null;
                         }, true);
                         if (trimmed.name) {
-                            def.name = trimmed.name;
-                            def.value = value = trimmed.value;
+                            def = make_node(AST_VarDef, def, {
+                                name: trimmed.name,
+                                value: value = trimmed.value,
+                            });
                             flush();
                         } else if (trimmed.value) {
                             side_effects.push(trimmed.value);
@@ -6722,19 +6724,18 @@ merge(Compressor.prototype, {
             }
             if (node instanceof AST_Assign) {
                 descend(node, tt);
-                if (node.left instanceof AST_Destructured) {
-                    var trimmed = trim_destructured(node.left, node.right, function(node) {
-                        return node;
-                    }, node.write_only === true);
-                    if (!trimmed.name) {
-                        if (trimmed.value) return trimmed.value;
-                        if (parent instanceof AST_Sequence && parent.tail_node() !== node) return List.skip;
-                        return make_node(AST_Number, node, { value: 0 });
-                    }
-                    node.left = trimmed.name;
-                    node.right = trimmed.value;
-                }
-                return node;
+                if (!(node.left instanceof AST_Destructured)) return node;
+                var trimmed = trim_destructured(node.left, node.right, function(node) {
+                    return node;
+                }, node.write_only === true);
+                if (trimmed.name) return make_node(AST_Assign, node, {
+                    operator: node.operator,
+                    left: trimmed.name,
+                    right: trimmed.value,
+                });
+                if (trimmed.value) return trimmed.value;
+                if (parent instanceof AST_Sequence && parent.tail_node() !== node) return List.skip;
+                return make_node(AST_Number, node, { value: 0 });
             }
             if (node instanceof AST_LabeledStatement && node.body instanceof AST_For) {
                 // Certain combination of unused name + side effect leads to invalid AST:

test/compress/destructured.js

@@ -2948,3 +2948,53 @@ issue_5074_method_pure_getters: {
     expect_stdout: "PASS"
     node_version: ">=6"
 }
+
+issue_5085_1: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+        toplevel: true,
+        unsafe: true,
+        unused: true,
+    }
+    input: {
+        var a = "PASS";
+        var [ b ] = [ 42, a ], c = b ? 0 : a = "FAIL";
+        console.log(a);
+    }
+    expect: {
+        var a = "PASS";
+        var b = [ 42 ][0];
+        b;
+        console.log(a);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=6"
+}
+
+issue_5085_2: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+        side_effects: true,
+        unsafe: true,
+        unused: true,
+    }
+    input: {
+        var a = "PASS";
+        (function(b) {
+            [ b ] = [ 42, a ];
+            var c = b ? 0 : a = "FAIL";
+        })();
+        console.log(a);
+    }
+    expect: {
+        var a = "PASS";
+        (function(b) {
+            b = [ 42 ][0];
+        })();
+        console.log(a);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=6"
+}