From 8c2b76eff98fbdfc7ed2154e75a86b028b25345f Mon Sep 17 00:00:00 2001
From: "Alex Lam S.L" <alexlamsl@gmail.com>
Date: Mon, 17 Jan 2022 15:18:42 +0000
Subject: [PATCH] enhance `side_effects` (#5304)

---
 lib/compress.js          | 57 ++++++++++++++++++++++++++++------------
 test/compress/spreads.js | 24 ++++++++++++++++-
 2 files changed, 63 insertions(+), 18 deletions(-)

diff --git a/lib/compress.js b/lib/compress.js
index 7729ffde..1807bbfe 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -5303,21 +5303,43 @@ Compressor.prototype.compress = function(node) {
         return map && map[prop];
     });
 
-    function spread_side_effects(exp) {
-        while ((exp = exp.tail_node()) instanceof AST_SymbolRef) {
-            exp = exp.fixed_value();
-            if (!exp) return true;
-        }
-        return !(exp instanceof AST_Array
-            || exp.TYPE == "Binary" && !lazy_op[exp.operator]
-            || exp instanceof AST_Constant
-            || exp instanceof AST_Lambda
-            || exp instanceof AST_Object && all(exp.properties, function(prop) {
+    // determine if object spread syntax may cause runtime exception
+    (function(def) {
+        def(AST_Node, return_false);
+        def(AST_Array, return_true);
+        def(AST_Assign, function() {
+            switch (this.operator) {
+              case "=":
+                return this.right.safe_to_spread();
+              case "&&=":
+              case "||=":
+              case "??=":
+                return this.left.safe_to_spread() && this.right.safe_to_spread();
+            }
+            return true;
+        });
+        def(AST_Binary, function() {
+            return !lazy_op[this.operator] || this.left.safe_to_spread() && this.right.safe_to_spread();
+        });
+        def(AST_Constant, return_true);
+        def(AST_Lambda, return_true);
+        def(AST_Object, function() {
+            return all(this.properties, function(prop) {
                 return !(prop instanceof AST_ObjectGetter || prop instanceof AST_Spread);
-            })
-            || exp instanceof AST_ObjectIdentity
-            || exp instanceof AST_Unary);
-    }
+            });
+        });
+        def(AST_ObjectIdentity, return_true);
+        def(AST_Sequence, function() {
+            return this.tail_node().safe_to_spread();
+        });
+        def(AST_SymbolRef, function() {
+            var fixed = this.fixed_value();
+            return fixed && fixed.safe_to_spread();
+        });
+        def(AST_Unary, return_true);
+    })(function(node, func) {
+        node.DEFMETHOD("safe_to_spread", func);
+    });
 
     // determine if expression has side effects
     (function(def) {
@@ -5329,7 +5351,8 @@ Compressor.prototype.compress = function(node) {
             });
         }
         function array_spread(node, compressor) {
-            return !node.expression.is_string(compressor) || node.expression.has_side_effects(compressor);
+            var exp = node.expression;
+            return !exp.is_string(compressor) || exp.has_side_effects(compressor);
         }
         def(AST_Node, return_true);
         def(AST_Array, function(compressor) {
@@ -5411,7 +5434,7 @@ Compressor.prototype.compress = function(node) {
         def(AST_Object, function(compressor) {
             return any(this.properties, compressor, function(node, compressor) {
                 var exp = node.expression;
-                return spread_side_effects(exp) || exp.has_side_effects(compressor);
+                return !exp.safe_to_spread() || exp.has_side_effects(compressor);
             });
         });
         def(AST_ObjectIdentity, return_false);
@@ -8387,7 +8410,7 @@ Compressor.prototype.compress = function(node) {
             });
             var values = trim(exprs, compressor, first_in_statement, function(node, compressor, first_in_statement) {
                 var exp = node.expression;
-                return spread_side_effects(exp) ? node : exp.drop_side_effect_free(compressor, first_in_statement);
+                return exp.safe_to_spread() ? exp.drop_side_effect_free(compressor, first_in_statement) : node;
             });
             if (!values) return null;
             if (values === exprs && !all(values, function(node) {
diff --git a/test/compress/spreads.js b/test/compress/spreads.js
index 13b12af8..49ebc2a8 100644
--- a/test/compress/spreads.js
+++ b/test/compress/spreads.js
@@ -241,7 +241,29 @@ drop_empty_call_2: {
     node_version: ">=6"
 }
 
-convert_hole: {
+convert_hole_array: {
+    options = {
+        spreads: true,
+    }
+    input: {
+        [ ...[ "PASS", , 42 ] ].forEach(function(a) {
+            console.log(a);
+        });
+    }
+    expect: {
+        [ "PASS", void 0, 42 ].forEach(function(a) {
+            console.log(a);
+        });
+    }
+    expect_stdout: [
+        "PASS",
+        "undefined",
+        "42",
+    ]
+    node_version: ">=6"
+}
+
+convert_hole_call: {
     options = {
         spreads: true,
     }