From 5fb7ae9a427ed2c48f72b5e4609006c9b94d0753 Mon Sep 17 00:00:00 2001
From: Justin Ridgewell <justin@ridgewell.name>
Date: Mon, 20 Sep 2021 01:15:15 -0400
Subject: [PATCH] Keep iife assignment in return statement for async/generators

Fixes #1029
---
 lib/compress/index.js      |  10 ++-
 test/compress/dead-code.js | 154 +++++++++++++++++++++++++++++++++++++
 2 files changed, 163 insertions(+), 1 deletion(-)

diff --git a/lib/compress/index.js b/lib/compress/index.js
index b03525510..c7ce05e35 100644
--- a/lib/compress/index.js
+++ b/lib/compress/index.js
@@ -3587,7 +3587,15 @@ function is_reachable(self, defs) {
         if (node instanceof AST_Scope && node !== self) {
             var parent = info.parent();
 
-            if (parent instanceof AST_Call && parent.expression === node) return;
+            if (
+                parent instanceof AST_Call
+                && parent.expression === node
+                // Async/Generators aren't guaranteed to sync evaluate all of
+                // their body steps, so it's possible they close over the variable.
+                && !(node.async || node.is_generator)
+            ) {
+                return;
+            }
 
             if (walk(node, find_ref)) return walk_abort;
 
diff --git a/test/compress/dead-code.js b/test/compress/dead-code.js
index f4bbba435..7b5492780 100644
--- a/test/compress/dead-code.js
+++ b/test/compress/dead-code.js
@@ -1253,3 +1253,157 @@ issue_718: {
         export {y}
     }
 }
+
+issue_1029_1: {
+    options = {
+        dead_code: true,
+    }
+    input: {
+        function asyncFn() {
+          let promise;
+          return promise = (async () => {
+            await true;
+            console.log(promise);
+          })()
+        }
+        asyncFn({});
+    }
+    expect: {
+        function asyncFn() {
+            let promise;
+            return promise = (async () => {
+                await true;
+                console.log(promise);
+            })();
+        }
+        asyncFn({});
+    }
+}
+
+issue_1029_2: {
+    options = {
+        dead_code: true,
+    }
+    input: {
+        function asyncFn() {
+          let promise;
+          return promise = (async () => {
+            console.log(promise);
+          })()
+        }
+        asyncFn({});
+    }
+    expect: {
+        function asyncFn() {
+            let promise;
+            return promise = (async () => {
+                console.log(promise);
+            })();
+        }
+        asyncFn({});
+    }
+}
+
+issue_1029_3: {
+    options = {
+        dead_code: true,
+    }
+    input: {
+        function genFn() {
+            let gen;
+            return gen = function*() {
+                console.log(gen);
+            }();
+        }
+        genFn({}).next();
+    }
+    expect: {
+        function genFn() {
+            let gen;
+            return gen = function* () {
+                console.log(gen);
+            }();
+        }
+        genFn({}).next();
+    }
+}
+
+issue_1029_4: {
+    options = {
+        dead_code: true,
+    }
+    input: {
+        function fn() {
+            let val
+            return val = function() {
+                console.log(val);
+                return {};
+            }();
+        }
+        fn();
+    }
+    expect: {
+        function fn() {
+            let val
+            return function() {
+                console.log(val);
+                return {};
+            }();
+        }
+        fn();
+    }
+}
+
+issue_1029_5: {
+    options = {
+        dead_code: true,
+    }
+    input: {
+        function fn() {
+            let val
+            return val = function() {
+                setTimeout(() => console.log(val));
+                return {};
+            }();
+        }
+        fn();
+    }
+    expect: {
+        function fn() {
+            let val
+            return val = function() {
+                setTimeout(() => console.log(val));
+                return {};
+            }();
+        }
+        fn();
+    }
+}
+
+issue_1029_6: {
+    options = {
+        dead_code: true,
+    }
+    input: {
+        function fn() {
+            let val
+            return val = function() {
+                setTimeout(() => {
+                    (() => console.log(val))();
+                })
+                return {};
+            }();
+        }
+        fn();
+    }
+    expect: {
+        function fn() {
+            let val
+            return val = function() {
+                setTimeout(() => console.log(val));
+                return {};
+            }();
+        }
+        fn();
+    }
+}