fix corner case in unused (#5190)

fixes #5189

lib/compress.js

@@ -7211,6 +7211,9 @@ merge(Compressor.prototype, {
                         if (!drop) break;
                         var sym = elements[0];
                         if (sym.has_side_effects(compressor)) break;
+                        if (value.has_side_effects(compressor) && sym.match_symbol(function(node) {
+                            return node instanceof AST_PropAccess;
+                        })) break;
                         value = make_node(AST_Sub, node, {
                             expression: value,
                             property: make_node(AST_Number, node, { value: 0 }),
@@ -7337,6 +7340,9 @@ merge(Compressor.prototype, {
                         var prop = properties[0];
                         if (prop.key instanceof AST_Node) break;
                         if (prop.value.has_side_effects(compressor)) break;
+                        if (value.has_side_effects(compressor) && prop.value.match_symbol(function(node) {
+                            return node instanceof AST_PropAccess;
+                        })) break;
                         value = make_node(AST_Sub, node, {
                             expression: value,
                             property: make_node_from_constant(prop.key, prop),

test/compress/destructured.js

@@ -3322,3 +3322,47 @@ issue_5168: {
     expect_stdout: "function"
     node_version: ">=6"
 }
+
+issue_5189_1: {
+    options = {
+        pure_getters: "strict",
+        reduce_vars: true,
+        side_effects: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a = 42;
+        [ a.p ] = a = "PASS";
+        console.log(a);
+    }
+    expect: {
+        var a;
+        [ a.p ] = a = "PASS";
+        console.log(a);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=6"
+}
+
+issue_5189_2: {
+    options = {
+        pure_getters: "strict",
+        reduce_vars: true,
+        side_effects: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a = 42;
+        ({ p: a.q } = a = "PASS");
+        console.log(a);
+    }
+    expect: {
+        var a;
+        ({ p: a.q } = a = "PASS");
+        console.log(a);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=6"
+}