fix corner case in collapse_vars (#4332)

fixes #4331

lib/compress.js

@@ -2047,6 +2047,7 @@ merge(Compressor.prototype, {
                     return (parent.tail_node() === node ? find_stop_value : find_stop_unused)(parent, level + 1);
                 }
                 if (parent instanceof AST_SimpleStatement) return find_stop_unused(parent, level + 1);
+                if (parent instanceof AST_Spread) return find_stop_value(parent, level + 1);
                 if (parent instanceof AST_Switch) {
                     if (parent.expression !== node) return node;
                     return find_stop_value(parent, level + 1);

test/compress/spread.js

@@ -399,3 +399,29 @@ issue_4329: {
     expect_stdout: "PASS"
     node_version: ">=8"
 }
+
+issue_4331: {
+    options = {
+        collapse_vars: true,
+        toplevel: true,
+    }
+    input: {
+        var a = "PASS", b;
+        console,
+        b = a;
+        (function() {
+            a++;
+        })(...a);
+        console.log(b);
+    }
+    expect: {
+        var a = "PASS", b;
+        console;
+        (function() {
+            a++;
+        })(...b = a);
+        console.log(b);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=8"
+}