fix corner case in `reduce_vars` & `unused` (#4465)

fixes #4464

3 files changed, 87 insertions, 6 deletions

diff --git a/lib/compress.js b/lib/compress.js
index ae66d140..97b6149b 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -363,6 +363,10 @@ merge(Compressor.prototype, {
         return orig.length == 1 && orig[0] instanceof AST_SymbolFunarg;
     }
 
+    function is_funarg(def) {
+        return def.orig[0] instanceof AST_SymbolFunarg || def.orig[1] instanceof AST_SymbolFunarg;
+    }
+
     function cross_scope(def, sym) {
         do {
             if (def === sym) return false;
@@ -371,9 +375,9 @@ merge(Compressor.prototype, {
     }
 
     function can_drop_symbol(ref, keep_lambda) {
-        var orig = ref.definition().orig;
-        if (ref.in_arg && (orig[0] instanceof AST_SymbolFunarg || orig[1] instanceof AST_SymbolFunarg)) return false;
-        return all(orig, function(sym) {
+        var def = ref.definition();
+        if (ref.in_arg && is_funarg(def)) return false;
+        return all(def.orig, function(sym) {
             return !(sym instanceof AST_SymbolConst || sym instanceof AST_SymbolLet
                 || keep_lambda && sym instanceof AST_SymbolLambda);
         });
@@ -541,7 +545,8 @@ merge(Compressor.prototype, {
             return compressor.option("unused")
                 && !def.scope.pinned()
                 && def.single_use !== false
-                && def.references.length - def.recursive_refs == 1;
+                && def.references.length - def.recursive_refs == 1
+                && !(is_funarg(def) && def.scope.uses_arguments);
         }
 
         function is_immutable(value) {
@@ -9331,7 +9336,7 @@ merge(Compressor.prototype, {
                         single_use = false;
                     } else if (fixed.name && fixed.name.definition() !== def) {
                         single_use = false;
-                    } else if (fixed.parent_scope !== self.scope.resolve() || def.orig[0] instanceof AST_SymbolFunarg) {
+                    } else if (fixed.parent_scope !== self.scope.resolve() || is_funarg(def)) {
                         single_use = fixed.is_constant_expression(self.scope);
                         if (single_use == "f") {
                             var scope = self.scope;
@@ -9414,7 +9419,7 @@ merge(Compressor.prototype, {
             if (fixed && (local || def.should_replace !== false)) {
                 var init;
                 if (fixed instanceof AST_This) {
-                    if (!(def.orig[0] instanceof AST_SymbolFunarg) && same_scope(def)) {
+                    if (!is_funarg(def) && same_scope(def)) {
                         init = fixed;
                     }
                 } else {
diff --git a/test/compress/drop-unused.js b/test/compress/drop-unused.js
index 3feb0183..ae1a6e5b 100644
--- a/test/compress/drop-unused.js
+++ b/test/compress/drop-unused.js
@@ -3125,3 +3125,74 @@ issue_4413: {
     }
     expect_stdout: "0"
 }
+
+issue_4464_1: {
+    options = {
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        function f(a) {
+            var a = function() {};
+            return [ arguments, a ];
+        }
+        console.log(typeof f()[1]);
+    }
+    expect: {
+        function f(a) {
+            a = function() {};
+            return [ arguments, a ];
+        }
+        console.log(typeof f()[1]);
+    }
+    expect_stdout: "function"
+}
+
+issue_4464_2: {
+    options = {
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        function f(a) {
+            var a = function() {};
+            return [ arguments, a ];
+        }
+        console.log(typeof f(42)[0][0]);
+    }
+    expect: {
+        function f(a) {
+            a = function() {};
+            return [ arguments, a ];
+        }
+        console.log(typeof f(42)[0][0]);
+    }
+    expect_stdout: "function"
+}
+
+issue_4464_3: {
+    options = {
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        (function a(a) {
+            var a = function() {};
+            return [ arguments[0], a ];
+        })(42).forEach(function(b) {
+            console.log(typeof b);
+        });
+    }
+    expect: {
+        (function(a) {
+            a = function() {};
+            return [ arguments[0], a ];
+        })(42).forEach(function(b) {
+            console.log(typeof b);
+        });
+    }
+    expect_stdout: [
+        "function",
+        "function",
+    ]
+}
diff --git a/test/reduce.js b/test/reduce.js
index f6b54141..48030b0f 100644
--- a/test/reduce.js
+++ b/test/reduce.js
@@ -352,6 +352,11 @@ module.exports = function reduce_test(testcase, minify_options, reduce_options)
                     }
                 }
             }
+            else if (node instanceof U.AST_Spread) {
+                node.start._permute++;
+                CHANGED = true;
+                return node.expression;
+            }
             else if (node instanceof U.AST_Switch) {
                 var expr = [
                     node.expression,                         // switch expression