fix corner case in unused (#5101)

fixes #5100

Author: alexlamsl

lib/compress.js

@@ -7301,12 +7301,11 @@ merge(Compressor.prototype, {
                     if (value === null) {
                         value = make_node(AST_Number, node, { value: 0 });
                     } else if (value) {
-                        if (value.may_throw_on_access(compressor, true)) value = make_sequence(node, [
-                            value,
-                            make_node(AST_Number, node, { value: 0 }),
-                        ]);
-                    } else if (node instanceof AST_DestructuredArray) {
-                        return make_node(AST_DestructuredArray, node, { elements: [] });
+                        if (value.tail_node().write_only === true || value.may_throw_on_access(compressor, true)) {
+                            value = make_node(AST_Array, node, {
+                                elements: value instanceof AST_Sequence ? value.expressions : [ value ],
+                            });
+                        }
                     }
                     return make_node(AST_DestructuredObject, node, { properties: [] });
                 }

test/compress/rests.js

@@ -1003,3 +1003,69 @@ issue_5089_2: {
     expect_stdout: "undefined"
     node_version: ">=8"
 }
+
+issue_5100_1: {
+    options = {
+        passes: 2,
+        pure_getters: "strict",
+        side_effects: true,
+        unused: true,
+    }
+    input: {
+        var a;
+        [ {
+            p: {},
+            ...a
+        } ] = [ {
+            p: {
+                q: a,
+            } = 42,
+            r: "PASS",
+        } ];
+        console.log(a.r);
+    }
+    expect: {
+        var a;
+        [ {
+            p: {},
+            ...a
+        } ] = [ {
+            p: [ a = 42["q"] ],
+            r: "PASS",
+        } ];
+        console.log(a.r);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=8"
+}
+
+issue_5100_2: {
+    options = {
+        passes: 2,
+        pure_getters: "strict",
+        side_effects: true,
+        unused: true,
+    }
+    input: {
+        var a;
+        [ {
+            p: {},
+            ...a
+        } ] = [ {
+            p: (console.log("PASS"), {
+                q: a,
+            } = 42),
+        } ];
+    }
+    expect: {
+        var a;
+        [ {
+            p: {},
+            ...a
+        } ] = [ {
+            p: [ console.log("PASS"), a = 42["q"] ],
+        } ];
+    }
+    expect_stdout: "PASS"
+    node_version: ">=10"
+}