introduce `spread` (#4346)

fixes #4345

4 files changed, 95 insertions, 48 deletions

diff --git a/README.md b/README.md
index bda1f02b..5718014d 100644
--- a/README.md
+++ b/README.md
@@ -751,6 +751,8 @@ to be `false` and all symbol names will be omitted.
   annotation `/*@__PURE__*/` or `/*#__PURE__*/` immediately precedes the call. For
   example: `/*@__PURE__*/foo();`
 
+- `spread` (default: `true`) -- flatten spread expressions.
+
 - `strings` (default: `true`) -- compact string concatenations.
 
 - `switches` (default: `true`) -- de-duplicate and remove unreachable `switch` branches
diff --git a/lib/compress.js b/lib/compress.js
index 21ecc5e5..1b5b8be6 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -84,6 +84,7 @@ function Compressor(options, false_by_default) {
         reduce_vars     : !false_by_default,
         sequences       : !false_by_default,
         side_effects    : !false_by_default,
+        spread          : !false_by_default,
         strings         : !false_by_default,
         switches        : !false_by_default,
         top_retain      : null,
@@ -9724,27 +9725,10 @@ merge(Compressor.prototype, {
         });
     });
 
-    function is_integer(key) {
-        return /^[0-9]+$/.test(key);
-    }
-
     OPT(AST_Spread, function(self, compressor) {
-        if (compressor.option("properties")) {
-            var exp = self.expression;
-            if (compressor.parent() instanceof AST_Object) {
-                if (exp instanceof AST_Object && all(exp.properties, function(node) {
-                    return node instanceof AST_ObjectKeyVal;
-                })) return List.splice(exp.properties.map(function(node) {
-                    var key = node.key;
-                    if (!(key instanceof AST_Node) && is_integer(key)) {
-                        node = node.clone();
-                        node.key = make_node(AST_Number, node, {
-                            value: +key
-                        });
-                    }
-                    return node;
-                }));
-            } else if (exp instanceof AST_Array) return List.splice(exp.elements.map(function(node) {
+        var exp = self.expression;
+        if (compressor.option("spread") && exp instanceof AST_Array && !(compressor.parent() instanceof AST_Object)) {
+            return List.splice(exp.elements.map(function(node) {
                 return node instanceof AST_Hole ? make_node(AST_Undefined, node).optimize(compressor) : node;
             }));
         }
@@ -10014,40 +9998,69 @@ merge(Compressor.prototype, {
     });
 
     OPT(AST_Object, function(self, compressor) {
-        if (!compressor.option("objects") || compressor.has_directive("use strict")) return self;
-        for (var i = self.properties.length; --i >= 0;) {
-            var prop = self.properties[i];
-            var key = prop.key;
-            if (key instanceof AST_Node) key = key.evaluate(compressor);
-            if (is_integer(key)) break;
-            if (key !== prop.key) prop.key = "" + key;
-        }
+        if (!compressor.option("objects")) return self;
+        var changed = false;
+        var computed_int = false;
+        var has_computed = false;
+        var keep_duplicate = compressor.has_directive("use strict");
         var keys = new Dictionary();
         var values = [];
         self.properties.forEach(function(prop) {
-            if (prop instanceof AST_ObjectKeyVal && typeof prop.key == "string") {
-                if (prop.value.has_side_effects(compressor)) flush();
-                keys.add(prop.key, prop.value);
+            if (!(prop instanceof AST_Spread)) return process(prop);
+            var exp = prop.expression;
+            if (exp instanceof AST_Object && all(exp.properties, function(node) {
+                return node instanceof AST_ObjectKeyVal;
+            })) {
+                changed = true;
+                has_computed = true;
+                exp.properties.forEach(process);
             } else {
                 flush();
                 values.push(prop);
             }
         });
         flush();
-        if (self.properties.length != values.length) return make_node(AST_Object, self, {
+        return changed ? make_node(AST_Object, self, {
             properties: values
-        });
-        return self;
+        }) :  self;
 
         function flush() {
-            keys.each(function(expressions, key) {
+            keys.each(function(props) {
+                if (props.length == 1) return values.push(props[0]);
+                changed = true;
+                var tail = keep_duplicate && props.pop();
                 values.push(make_node(AST_ObjectKeyVal, self, {
-                    key: key,
-                    value: make_sequence(self, expressions)
+                    key: props[0].key,
+                    value: make_sequence(self, props.map(function(prop) {
+                        return prop.value;
+                    }))
                 }));
+                if (tail) values.push(tail);
             });
             keys = new Dictionary();
         }
+
+        function process(prop) {
+            var key = prop.key;
+            if (key instanceof AST_Node) {
+                has_computed = true;
+                key = key.evaluate(compressor);
+                if (key !== prop.key) key = prop.key = "" + key;
+            }
+            if (prop instanceof AST_ObjectKeyVal && typeof key == "string") {
+                if (prop.value.has_side_effects(compressor)) flush();
+                keys.add(key, prop);
+            } else {
+                flush();
+                values.push(prop);
+            }
+            if (has_computed && !computed_int && typeof key == "string" && /^[0-9]+$/.test(key)) {
+                computed_int = true;
+                prop.key = make_node(AST_Number, prop, {
+                    value: +key
+                });
+            }
+        }
     });
 
     OPT(AST_Return, function(self, compressor) {
diff --git a/test/compress/objects.js b/test/compress/objects.js
index 51b7bcc4..5d3653d0 100644
--- a/test/compress/objects.js
+++ b/test/compress/objects.js
@@ -45,8 +45,8 @@ duplicate_key_strict: {
         "use strict";
         var o = {
             a: 1,
-            b: 2,
             a: 3,
+            b: 2,
         };
         for (var k in o)
             console.log(k, o[k]);
@@ -323,8 +323,8 @@ issue_4269_3: {
     }
     expect: {
         console.log({
-            ["foo"]: "bar",
-            get 42() {
+            foo: "bar",
+            get [42]() {
                 return "FAIL";
             },
             42: "PASS",
@@ -353,8 +353,8 @@ issue_4269_4: {
             get 42() {
                 return "FAIL";
             },
-            ["foo"]: "bar",
-            42: "PASS",
+            foo: "bar",
+            [42]: "PASS",
         }[42]);
     }
     expect_stdout: "PASS"
diff --git a/test/compress/spread.js b/test/compress/spread.js
index 6763d4f7..aaa8a7f7 100644
--- a/test/compress/spread.js
+++ b/test/compress/spread.js
@@ -105,8 +105,8 @@ dont_inline: {
 
 do_inline: {
     options = {
-        properties: true,
         inline: true,
+        spread: true,
     }
     input: {
         console.log(function(a) {
@@ -144,8 +144,8 @@ drop_empty_call_1: {
 
 drop_empty_call_2: {
     options = {
-        properties: true,
         side_effects: true,
+        spread: true,
     }
     input: {
         (function() {})(...[ console.log("PASS") ]);
@@ -159,7 +159,7 @@ drop_empty_call_2: {
 
 convert_hole: {
     options = {
-        properties: true,
+        spread: true,
     }
     input: {
         console.log(...[ "PASS", , 42 ]);
@@ -295,7 +295,8 @@ keep_getter: {
 
 keep_accessor: {
     options = {
-        properties: true,
+        objects: true,
+        spread: true,
     }
     input: {
         var o = {
@@ -372,7 +373,8 @@ unused_var_side_effects: {
 
 issue_4329: {
     options = {
-        properties: true,
+        objects: true,
+        spread: true,
     }
     input: {
         console.log({
@@ -423,7 +425,7 @@ issue_4331: {
         console.log(b);
     }
     expect_stdout: "PASS"
-    node_version: ">=8"
+    node_version: ">=6"
 }
 
 issue_4342: {
@@ -447,3 +449,33 @@ issue_4342: {
     expect_stdout: "PASS"
     node_version: ">=6"
 }
+
+issue_4345: {
+    options = {
+        objects: true,
+        spread: true,
+    }
+    input: {
+        console.log({
+            ...{
+                get 42() {
+                    return "FAIL";
+                },
+                ...{},
+                42: "PASS",
+            },
+        }[42]);
+    }
+    expect: {
+        console.log({
+            ...{
+                get 42() {
+                    return "FAIL";
+                },
+                [42]: "PASS",
+            },
+        }[42]);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=8"
+}