fix corner case in conditionals (#4492)

lib/compress.js

@@ -9843,10 +9843,17 @@ merge(Compressor.prototype, {
             && !condition.has_side_effects(compressor)
             && !consequent.expression.has_side_effects(compressor)) {
             var node = consequent.clone();
-            node.args[arg_index] = make_node(AST_Conditional, self, {
+            var arg = consequent.args[arg_index];
+            node.args[arg_index] = arg instanceof AST_Spread ? make_node(AST_Spread, self, {
+                expression: make_node(AST_Conditional, self, {
+                    condition: condition,
+                    consequent: arg.expression,
+                    alternative: alternative.args[arg_index].expression,
+                }),
+            }) : make_node(AST_Conditional, self, {
                 condition: condition,
-                consequent: consequent.args[arg_index],
-                alternative: alternative.args[arg_index]
+                consequent: arg,
+                alternative: alternative.args[arg_index],
             });
             return node;
         }

test/compress/spread.js

@@ -85,6 +85,28 @@ collapse_vars_4: {
     node_version: ">=6"
 }
 
+conditionals_farg: {
+    options = {
+        conditionals: true,
+    }
+    input: {
+        function log(msg) {
+            console.log(msg);
+        }
+        var a = 42, b = [ "PASS" ], c = [ "FAIL" ];
+        a ? log(...b) : log(...c);
+    }
+    expect: {
+        function log(msg) {
+            console.log(msg);
+        }
+        var a = 42, b = [ "PASS" ], c = [ "FAIL" ];
+        log(...a ? b : c);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=6"
+}
+
 dont_inline: {
     options = {
         inline: true,