fix corner cases in `unused` (#5073)

fixes #5071

2 files changed, 56 insertions, 19 deletions

diff --git a/lib/compress.js b/lib/compress.js
index 28f127d5..e3816fa0 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -6712,7 +6712,7 @@ merge(Compressor.prototype, {
                 if (node.left instanceof AST_Destructured) {
                     var trimmed = trim_destructured(node.left, node.right, function(node) {
                         return node;
-                    }, node.write_only);
+                    }, 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;
@@ -7052,21 +7052,23 @@ merge(Compressor.prototype, {
                         value = value.fixed_value();
                     }
                     var values = value instanceof AST_Array && value.elements;
-                    var elements = [], newValues = [], pos = 0;
+                    var elements = [], newValues = drop && [], pos = 0;
                     node.elements.forEach(function(element, index) {
                         value = values && values[index];
                         if (value instanceof AST_Hole) {
                             value = null;
                         } else if (value instanceof AST_Spread) {
-                            newValues.length = pos;
-                            fill_holes(save_value, newValues);
-                            [].push.apply(newValues, values.slice(index));
-                            save_value.elements = newValues;
+                            if (drop) {
+                                newValues.length = pos;
+                                fill_holes(save_value, newValues);
+                                [].push.apply(newValues, values.slice(index));
+                                save_value.elements = newValues;
+                            }
                             value = values = false;
                         }
                         element = element.transform(trimmer);
                         if (element) elements[pos] = element;
-                        if (value) newValues[pos] = value;
+                        if (drop && value) newValues[pos] = value;
                         if (element || value || !drop || !values) pos++;
                     });
                     value = values && make_node(AST_Array, save_value, {
@@ -7079,21 +7081,23 @@ merge(Compressor.prototype, {
                         drop = was_drop;
                         if (node.rest) elements.length = pos;
                     }
-                    if (drop && value && !node.rest) value = value.drop_side_effect_free(compressor);
-                    if (value instanceof AST_Array) {
-                        value = value.elements;
-                    } else if (value instanceof AST_Sequence) {
-                        value = value.expressions;
-                    } else if (value) {
-                        value = [ value ];
-                    }
-                    if (value && value.length) {
-                        newValues.length = pos;
-                        [].push.apply(newValues, value);
+                    if (drop) {
+                        if (value && !node.rest) value = value.drop_side_effect_free(compressor);
+                        if (value instanceof AST_Array) {
+                            value = value.elements;
+                        } else if (value instanceof AST_Sequence) {
+                            value = value.expressions;
+                        } else if (value) {
+                            value = [ value ];
+                        }
+                        if (value && value.length) {
+                            newValues.length = pos;
+                            [].push.apply(newValues, value);
+                        }
                     }
                     value = save_value;
                     drop = save_drop;
-                    if (values && value instanceof AST_Array) {
+                    if (values && newValues) {
                         fill_holes(value, newValues);
                         value.elements = newValues;
                     }
diff --git a/test/compress/destructured.js b/test/compress/destructured.js
index 7db2d238..45724b1a 100644
--- a/test/compress/destructured.js
+++ b/test/compress/destructured.js
@@ -2783,3 +2783,36 @@ issue_5017: {
     expect_stdout: "PASS"
     node_version: ">=6"
 }
+
+issue_5071_1: {
+    options = {
+        unused: true,
+    }
+    input: {
+        var a;
+        console.log(([ , a ] = [ "PA", , ]).join("SS"));
+    }
+    expect: {
+        var a;
+        console.log(([ , a ] = [ "PA", , ]).join("SS"));
+    }
+    expect_stdout: "PASS"
+    node_version: ">=6"
+}
+
+issue_5071_2: {
+    options = {
+        pure_getters: "strict",
+        unused: true,
+    }
+    input: {
+        var a;
+        ([ a ] = []).p = console.log("PASS");
+    }
+    expect: {
+        var a;
+        ([ a ] = []).p = console.log("PASS");
+    }
+    expect_stdout: "PASS"
+    node_version: ">=6"
+}