WIP: fix: don't drop side effects of default param assignments

Closes terser#880

lib/compress/index.js

@@ -2463,7 +2463,49 @@ def_optimize(AST_Call, function(self, compressor) {
         }).optimize(compressor);
     }
 
-    const can_drop_this_call = is_regular_func && compressor.option("side_effects") && fn.body.every(is_empty);
+    function parameter_defaults_have_side_effects() {
+        return fn.argnames.some((param, ind) => {
+            if (!(param instanceof AST_DefaultAssign)) {
+                return false;
+            }
+
+            const arg = self.args[ind];
+            const default_param_assignment_wont_be_performed =
+                arg
+                && (
+                    // If the argument evaluates to `undefined`, default assignment is also performed.
+                    // This checks whether the argument is guaranteed to NOT evaluate to `undefined`.
+
+                    // TODO this is too narrow. Consider things like objects, functions, function calls
+                    // and all the other stuff.
+
+                    // Not sure if this doesn't give false positives.
+                    // // Keep in mind that the fact that `is_undefined()` alone returned `false` doesn't mean that
+                    // // the argument can't evaluate to `undefined` at runtime, so we also need `is_constant()`
+                    // // to make it so.
+                    // arg.is_constant()
+                    // && !is_undefined(arg)
+
+                    arg.is_boolean()
+                    || arg.is_number()
+                    || arg.is_string()
+                );
+            if (default_param_assignment_wont_be_performed) {
+                return false;
+            }
+
+            // Keep in mind that this could be a destructuring assignment, so
+            // `!param.right.has_side_effects(compressor)` is not enough.
+            if (!param.has_side_effects(compressor)) {
+                return false;
+            }
+
+            return true;
+        });
+    }
+
+    const can_drop_this_call = is_regular_func && compressor.option("side_effects") && fn.body.every(is_empty)
+        && !parameter_defaults_have_side_effects();
     if (can_drop_this_call) {
         var args = self.args.concat(make_node(AST_Undefined, self));
         return make_sequence(self, args).optimize(compressor);

test/compress/test.js

@@ -0,0 +1,214 @@
+// dsa: {
+//     options = {
+//         toplevel: true,
+//         unused: true,
+//         reduce_vars: true,
+//         side_effects: true,
+
+//         inline: true,
+//     }
+//     input: {
+//         // function cause_side_effect(a = console.log("side effect")) {}
+//         // cause_side_effect();
+
+//         function cause_side_effect(
+//             // {a} = {a:console.log("side effect 1")},
+//             // p2 = console.log("side effect 2"),
+//             // p3 = console.log("side effect 3"),
+
+//             p1 = 2,
+//             p2 = (()=>{})(),
+//         ) {}
+//         cause_side_effect(undefined, 2);
+
+//         // function dummy() {}
+//         // dummy(cause_side_effect());
+//     }
+//     expect: {
+//         console.log("side effect 1");
+//         console.log("side effect 2");
+//         console.log("side effect 3");
+//     }
+// }
+// asd: {
+//     options = {
+//         toplevel: true,
+//         unused: true,
+//         reduce_vars: true,
+//         side_effects: true,
+
+//         inline: true,
+//         // passes: 10,
+
+//         // collapse_vars: true,
+//         // conditionals: true,
+//         // evaluate: true,
+//         // inline: true,
+//         // passes: 3,
+//         // reduce_funcs: true,
+//         // reduce_vars: true,
+//         // sequences: true,
+//         // unused: true,
+//     }
+//     input: {
+//         // function cause_side_effect(a = console.log("side effect")) {}
+//         // cause_side_effect();
+
+//         function cause_side_effect(
+//             p1 = console.log("side effect 1"),
+//             p2 = console.log("side effect 2"),
+//             p3 = console.log("side effect 3"),
+//         ) {}
+//         cause_side_effect(undefined, 2);
+
+//         // function dummy() {}
+//         // dummy(cause_side_effect());
+//     }
+//     expect: {
+//         console.log("side effect 1");
+//         console.log("side effect 2");
+//         console.log("side effect 3");
+//     }
+// }
+simple: {
+    options = {
+        toplevel: true,
+        unused: true,
+        reduce_vars: true,
+        side_effects: true,
+        // passes: 5
+    }
+    input: {
+        function cause_side_effect(p = console.log("side effect")) {}
+
+        cause_side_effect();
+        // If the argument evaluates to `undefined`, default assignment is also performed.
+        cause_side_effect(undefined);
+        // If it is unknown what the argument evaluates to, keep.
+        // cause_side_effect(Math.random() > 0.5 ? undefined : true);
+        cause_side_effect(id());
+        cause_side_effect(id);
+    }
+    expect: {
+        function cause_side_effect(p = console.log("side effect")) {}
+
+        cause_side_effect();
+        cause_side_effect(void 0);
+        cause_side_effect(id());
+        cause_side_effect(id);
+    }
+    // TODO when I uncomment this it starts saying "failed running reminified input" and producing incorrect output.
+    // expect_stdout: [
+    //     "side effect",
+    //     "side effect",
+    //     "side effect",
+    // ]
+}
+drop_if_assignment_not_used: {
+    options = {
+        toplevel: true,
+        unused: true,
+        reduce_vars: true,
+        side_effects: true,
+    }
+    input: {
+        function cause_side_effect(p = console.log("side effect")) {}
+
+        cause_side_effect("a");
+        cause_side_effect(null);
+        cause_side_effect({ a: 1 });
+        cause_side_effect(() => 1);
+    }
+    expect: {}
+}
+// undefined_as_argument: {
+//     options = {
+//         toplevel: true,
+//         unused: true,
+//         reduce_vars: true,
+//         side_effects: true,
+//     }
+//     input: {
+//         function cause_side_effect(p = console.log("side effect")) {}
+//         cause_side_effect(undefined);
+//     }
+//     expect: {
+//         console.log("side effect")
+//     }
+// }
+no_false_positives: {
+    options = {
+        toplevel: true,
+        unused: true,
+        reduce_vars: true,
+        side_effects: true,
+    }
+    input: {
+        function no_side_effects(
+            p1,
+            p2 = "a",
+            p3 = (() => {})(),
+            p4 = undefined,
+            p5 = [p1, p2, p3, p4],
+            p6,
+            ...rest
+        ) {
+            // return [p1, p2, p3, p4, p5, p6, ...rest];
+        }
+        no_side_effects();
+    }
+    expect: {}
+}
+object_destructuring_getter_side_effect: {
+    options = {
+        toplevel: true,
+        unused: true,
+        reduce_vars: true,
+        side_effects: true,
+
+        pure_getters: false,
+    }
+    input: {
+        // Evaluating the object in itself is fine, but destructuring it can cause a side effect.
+        const obj = { get a() { console.log("side effect") } };
+        function cause_side_effect(
+            // {a, b} = { a: 1, b: 2 },
+            {a} = obj,
+        ) {}
+        cause_side_effect();
+    }
+    // expect: {
+    //     console.log("side effect")
+    // }
+    expect_stdout: [
+        "side effect",
+    ]
+}
+array_destructuring_getter_side_effect: {
+    options = {
+        toplevel: true,
+        unused: true,
+        reduce_vars: true,
+        side_effects: true,
+
+        pure_getters: false,
+    }
+    input: {
+        const arr = [0, 0, 0];
+        arr.__defineGetter__(0, function () {
+            console.log("side effect");
+            return 1;
+        })
+        function cause_side_effect(
+            // {a, b} = { a: 1, b: 2 },
+            [a, b] = arr,
+        ) {}
+        cause_side_effect();
+    }
+    // expect: {
+    //     console.log("side effect")
+    // }
+    expect_stdout: [
+        "side effect",
+    ]
+}