fix corner case in unused (#5668)

fixes #5663

lib/compress.js

@@ -7937,7 +7937,14 @@ Compressor.prototype.compress = function(node) {
                         drop = false;
                         value = value.fixed_value();
                     }
-                    var values = value instanceof AST_Array && value.elements;
+                    var native, values;
+                    if (value instanceof AST_Array) {
+                        native = true;
+                        values = value.elements;
+                    } else {
+                        native = value && value.is_string(compressor);
+                        values = false;
+                    }
                     var elements = [], newValues = drop && [], pos = 0;
                     node.elements.forEach(function(element, index) {
                         value = values && values[index];
@@ -7988,8 +7995,9 @@ Compressor.prototype.compress = function(node) {
                         value = value.clone();
                         value.elements = newValues;
                     }
-                    if (!node.rest && (value instanceof AST_Array
-                        || value && value.is_string(compressor))) switch (elements.length) {
+                    if (!native) {
+                        elements.length = node.elements.length;
+                    } else if (!node.rest) switch (elements.length) {
                       case 0:
                         if (node === root) break;
                         if (drop) value = value.drop_side_effect_free(compressor);

test/compress/default-values.js

@@ -2713,7 +2713,7 @@ issue_5533_2_drop_fargs: {
         try {
             (function() {
                 for (;;) {
-                    var [ [] = [] ] = [];
+                    var [ [ , ] = [] ] = [];
                     throw "PASS";
                 }
             })();

test/compress/destructured.js

@@ -1306,7 +1306,7 @@ keep_reference: {
     }
     expect: {
         var a = [ {}, 42 ];
-        var [ b ] = a;
+        var b = a[0];
         console.log(a[0] === b ? "PASS" : "FAIL");
     }
     expect_stdout: "PASS"

test/compress/yields.js

@@ -1735,3 +1735,37 @@ issue_5576: {
     ]
     node_version: ">=10"
 }
+
+issue_5663: {
+    options = {
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var [ , a ] = function*() {
+            console.log("foo");
+            yield console.log("bar");
+            console.log("baz");
+            yield console.log("moo");
+            console.log("moz");
+            yield FAIL;
+        }();
+    }
+    expect: {
+        var [ , , ] = function*() {
+            console.log("foo");
+            yield console.log("bar");
+            console.log("baz");
+            yield console.log("moo");
+            console.log("moz");
+            yield FAIL;
+        }();
+    }
+    expect_stdout: [
+        "foo",
+        "bar",
+        "baz",
+        "moo",
+    ]
+    node_version: ">=6"
+}