fix corner case in unused (#5361)

fixes #5360

lib/compress.js

@@ -7734,16 +7734,37 @@ Compressor.prototype.compress = function(node) {
                 return prop.key instanceof AST_Node && prop.key.has_side_effects(compressor);
             }
 
+            function clear_write_only(node) {
+                if (node instanceof AST_Assign) {
+                    node.write_only = false;
+                    clear_write_only(node.right);
+                } else if (node instanceof AST_Binary) {
+                    if (!lazy_op[node.operator]) return;
+                    clear_write_only(node.left);
+                    clear_write_only(node.right);
+                } else if (node instanceof AST_Conditional) {
+                    clear_write_only(node.consequent);
+                    clear_write_only(node.alternative);
+                } else if (node instanceof AST_Sequence) {
+                    clear_write_only(node.tail_node());
+                } else if (node instanceof AST_Unary) {
+                    node.write_only = false;
+                }
+            }
+
             function retain_lhs(node) {
                 if (node instanceof AST_DefaultValue) return retain_lhs(node.name);
                 if (node instanceof AST_Destructured) {
                     if (value === null) {
                         value = make_node(AST_Number, node, { value: 0 });
-                    } else if (value && (value.tail_node().write_only === true
-                        || value.may_throw_on_access(compressor, true))) {
+                    } else if (value) {
+                        if (value.may_throw_on_access(compressor, true)) {
                             value = make_node(AST_Array, node, {
                                 elements: value instanceof AST_Sequence ? value.expressions : [ value ],
                             });
+                        } else {
+                            clear_write_only(value);
+                        }
                     }
                     return make_node(AST_DestructuredObject, node, { properties: [] });
                 }

test/compress/rests.js

@@ -1049,7 +1049,9 @@ issue_5100_1: {
             p: {},
             ...a
         } = [ {
-            p: [ a = 42["q"] ],
+            p: {
+                q: a,
+            } = 42,
             r: "PASS",
         } ][0]);
         console.log(a.r);
@@ -1082,7 +1084,9 @@ issue_5100_2: {
             p: {},
             ...a
         } = [ {
-            p: [ console.log("PASS"), a = 42["q"] ],
+            p: (console.log("PASS"), {
+                q: a,
+            } = 42),
         } ][0]);
     }
     expect_stdout: "PASS"
@@ -1267,3 +1271,31 @@ issue_5246_3: {
     expect_stdout: "PASS"
     node_version: ">=6"
 }
+
+issue_5360: {
+    options = {
+        keep_fargs: false,
+        pure_getters: "strict",
+        unused: true,
+    }
+    input: {
+        var a;
+        console.log(function({ p: {}, ...b }) {
+            return b.q;
+        }({
+            p: ~a && ([ a ] = []),
+            q: "PASS",
+        }));
+    }
+    expect: {
+        var a;
+        console.log(function({ p: {}, ...b }) {
+            return b.q;
+        }({
+            p: ~a && ([ a ] = []),
+            q: "PASS",
+        }));
+    }
+    expect_stdout: "PASS"
+    node_version: ">=8.3.0"
+}