diff --git a/lib/compress.js b/lib/compress.js
index 49f2cd39..587f0888 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -1665,6 +1665,18 @@ Compressor.prototype.compress = function(node) {
         }
     }
 
+    function find_try(compressor, level, node, scope, may_throw, sync) {
+        for (var parent; parent = compressor.parent(level++); node = parent) {
+            if (parent === scope) return false;
+            if (sync && parent instanceof AST_Lambda) {
+                if (parent.name || is_async(parent) || is_generator(parent)) return true;
+            } else if (parent instanceof AST_Try) {
+                if (parent.bfinally && parent.bfinally !== node) return true;
+                if (may_throw && parent.bcatch && parent.bcatch !== node) return true;
+            }
+        }
+    }
+
     var identifier_atom = makePredicate("Infinity NaN undefined");
     function is_lhs_read_only(lhs, compressor) {
         if (lhs instanceof AST_ObjectIdentity) return true;
@@ -2585,7 +2597,7 @@ Compressor.prototype.compress = function(node) {
                         });
                     }
                     if (node instanceof AST_DestructuredObject) {
-                        if (!value.is_defined(compressor)) return true;
+                        if (value.may_throw_on_access(compressor)) return true;
                         return !all(node.properties, function(prop) {
                             if (prop.key instanceof AST_Node && reject(prop.key)) return false;
                             return !may_throw_arg(reject, prop.value);
@@ -2608,11 +2620,18 @@ Compressor.prototype.compress = function(node) {
                     })) {
                     var fn_strict = fn.in_strict_mode(compressor)
                         && !fn.parent_scope.resolve(true).in_strict_mode(compressor);
-                    var has_await = is_async(fn) ? function(node) {
-                        return node instanceof AST_Symbol && node.name == "await";
-                    } : function(node) {
-                        return node instanceof AST_Await && !tw.find_parent(AST_Scope);
-                    };
+                    var check_arg = false, has_await;
+                    if (is_async(fn)) {
+                        check_arg = true;
+                        has_await = function(node) {
+                            return node instanceof AST_Symbol && node.name == "await";
+                        };
+                    } else {
+                        check_arg = find_try(compressor, 1, iife, null, true, true);
+                        has_await = function(node) {
+                            return node instanceof AST_Await && !tw.find_parent(AST_Scope);
+                        };
+                    }
                     var arg_scope = null;
                     var tw = new TreeWalker(function(node, descend) {
                         if (!arg) return true;
@@ -2645,18 +2664,21 @@ Compressor.prototype.compress = function(node) {
                     for (var i = fn.argnames.length; --i >= 0;) {
                         var sym = fn.argnames[i];
                         var arg = args[i];
-                        var value;
+                        var value = null;
                         if (sym instanceof AST_DefaultValue) {
                             value = sym.value;
                             sym = sym.name;
                             args[len + i] = value;
                         }
                         if (sym instanceof AST_Destructured) {
-                            if (!may_throw_arg(function(node) {
+                            if (check_arg && may_throw_arg(function(node) {
                                 return node.has_side_effects(compressor);
-                            }, sym, arg)) continue;
-                            candidates.length = 0;
-                            break;
+                            }, sym, arg)) {
+                                candidates.length = 0;
+                                break;
+                            }
+                            args[len + i] = fn.argnames[i];
+                            continue;
                         }
                         if (names.has(sym.name)) continue;
                         names.set(sym.name, true);
@@ -12656,15 +12678,7 @@ Compressor.prototype.compress = function(node) {
             self.right = make_node(AST_Null, right);
             var may_throw = node.may_throw(compressor);
             self.right = right;
-            for (var parent; parent = compressor.parent(level++); node = parent) {
-                if (parent === scope) return false;
-                if (sync && parent instanceof AST_Lambda) {
-                    if (parent.name || is_async(parent) || is_generator(parent)) return true;
-                } else if (parent instanceof AST_Try) {
-                    if (parent.bfinally && parent.bfinally !== node) return true;
-                    if (may_throw && parent.bcatch && parent.bcatch !== node) return true;
-                }
-            }
+            return find_try(compressor, level, node, scope, may_throw, sync);
         }
 
         function strip_assignment(def) {
diff --git a/test/compress/awaits.js b/test/compress/awaits.js
index 5fee9e02..220f6fae 100644
--- a/test/compress/awaits.js
+++ b/test/compress/awaits.js
@@ -1046,6 +1046,60 @@ collapse_vars_3: {
     node_version: ">=8"
 }
 
+collapse_funarg_1: {
+    options = {
+        collapse_vars: true,
+        unused: true,
+    }
+    input: {
+        A = "FAIL";
+        var a = "PASS";
+        (async function({}, b) {
+            return b;
+        })(null, A = a);
+        console.log(A);
+    }
+    expect: {
+        A = "FAIL";
+        var a = "PASS";
+        (async function({}, b) {
+            return b;
+        })(null, A = a);
+        console.log(A);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=8"
+}
+
+collapse_funarg_2: {
+    options = {
+        collapse_vars: true,
+        unused: true,
+    }
+    input: {
+        A = "FAIL";
+        B = "PASS";
+        (async function() {
+            console.log(function({}, a) {
+                return a;
+            }(null, A = B));
+        })();
+        console.log(A);
+    }
+    expect: {
+        A = "FAIL";
+        B = "PASS";
+        (async function() {
+            console.log(function({}, a) {
+                return a;
+            }(null, A = B));
+        })();
+        console.log(A);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=8"
+}
+
 collapse_property_lambda: {
     options = {
         collapse_vars: true,
diff --git a/test/compress/default-values.js b/test/compress/default-values.js
index bb1a1133..d11a256e 100644
--- a/test/compress/default-values.js
+++ b/test/compress/default-values.js
@@ -1170,6 +1170,49 @@ mangle_arrow_2_toplevel: {
     node_version: ">=6.9.3"
 }
 
+collapse_preceding_simple_arg: {
+    options = {
+        collapse_vars: true,
+        unused: true,
+    }
+    input: {
+        var a = "foo";
+        console.log(function(b, c = "bar") {
+            return b + c;
+        }(a, a));
+    }
+    expect: {
+        var a = "foo";
+        console.log(function(b, c = "bar") {
+            return a + c;
+        }(0, a));
+    }
+    expect_stdout: "foofoo"
+    node_version: ">=6"
+}
+
+drop_preceding_simple_arg: {
+    options = {
+        collapse_vars: true,
+        keep_fargs: false,
+        unused: true,
+    }
+    input: {
+        var a = "foo";
+        console.log(function(b, c = "bar") {
+            return b + c;
+        }(a, a));
+    }
+    expect: {
+        var a = "foo";
+        console.log(function(c = "bar") {
+            return a + c;
+        }(a));
+    }
+    expect_stdout: "foofoo"
+    node_version: ">=6"
+}
+
 issue_4444: {
     options = {
         collapse_vars: true,
diff --git a/test/compress/destructured.js b/test/compress/destructured.js
index 4f615e92..4cef240b 100644
--- a/test/compress/destructured.js
+++ b/test/compress/destructured.js
@@ -472,6 +472,93 @@ funarg_collapse_vars_3: {
     node_version: ">=6"
 }
 
+funarg_collapse_vars_4: {
+    options = {
+        collapse_vars: true,
+        unused: true,
+    }
+    input: {
+        var a = "PASS";
+        (function(b, { log: c }) {
+            c(b);
+        })(a, console);
+    }
+    expect: {
+        var a = "PASS";
+        (function(b, { log: c }) {
+            c(a);
+        })(0, console);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=6"
+}
+
+funarg_collapse_vars_5: {
+    options = {
+        collapse_vars: true,
+        unused: true,
+    }
+    input: {
+        A = "FAIL";
+        B = "PASS";
+        try {
+            console.log(function({}, a) {
+                return a;
+            }(null, A = B));
+        } catch (e) {}
+        console.log(A);
+    }
+    expect: {
+        A = "FAIL";
+        B = "PASS";
+        try {
+            console.log(function({}, a) {
+                return a;
+            }(null, A = B));
+        } catch (e) {}
+        console.log(A);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=6"
+}
+
+funarg_collapse_vars_6: {
+    options = {
+        collapse_vars: true,
+        unused: true,
+    }
+    input: {
+        A = "FAIL";
+        B = "PASS";
+        function f() {
+            console.log(function({}, a) {
+                return a;
+            }(null, A = B));
+        }
+        try {
+            f();
+        } catch (e) {
+            console.log(A);
+        }
+    }
+    expect: {
+        A = "FAIL";
+        B = "PASS";
+        function f() {
+            console.log(function({}, a) {
+                return a;
+            }(null, A = B));
+        }
+        try {
+            f();
+        } catch (e) {
+            console.log(A);
+        }
+    }
+    expect_stdout: "PASS"
+    node_version: ">=6"
+}
+
 funarg_reduce_vars_1: {
     options = {
         reduce_vars: true,