fix corner case in awaits (#5828)

fixes #5791

lib/compress.js

@@ -3964,12 +3964,19 @@ Compressor.prototype.compress = function(node) {
             for (var index = statements.length; --index >= 0;) {
                 var stat = statements[index];
                 if (!(stat instanceof AST_SimpleStatement)) break;
-                var node = stat.body;
+                var node = stat.body.tail_node();
                 if (!(node instanceof AST_Await)) break;
                 var exp = node.expression;
                 if (!needs_enqueuing(compressor, exp)) break;
                 changed = true;
                 exp = exp.drop_side_effect_free(compressor, true);
+                if (stat.body instanceof AST_Sequence) {
+                    var expressions = stat.body.expressions.slice();
+                    expressions.pop();
+                    if (exp) expressions.push(exp);
+                    stat.body = make_sequence(stat.body, expressions);
+                    break;
+                }
                 if (exp) {
                     stat.body = exp;
                     break;
@@ -11752,7 +11759,16 @@ Compressor.prototype.compress = function(node) {
             return !lazy_op[node.operator]
                 || needs_enqueuing(compressor, node.left) && needs_enqueuing(compressor, node.right);
         }
-        if (node instanceof AST_Call) return is_async(node.expression);
+        if (node instanceof AST_Call) {
+            if (!is_async(node.expression)) return false;
+            var has_await = false;
+            walk_body(node.expression, new TreeWalker(function(expr) {
+                if (has_await) return true;
+                if (expr instanceof AST_Await) return has_await = true;
+                if (expr !== node && expr instanceof AST_Scope) return true;
+            }));
+            return !has_await;
+        }
         if (node instanceof AST_Conditional) {
             return needs_enqueuing(compressor, node.consequent) && needs_enqueuing(compressor, node.alternative);
         }

test/compress/awaits.js

@@ -3642,3 +3642,66 @@ issue_5692_2: {
     ]
     node_version: ">=8"
 }
+
+issue_5791: {
+    options = {
+        awaits: true,
+        reduce_funcs: true,
+        reduce_vars: true,
+        side_effects: true,
+        unused: true,
+    }
+    input: {
+        (async function() {
+            async function f() {
+                try {
+                    await {
+                        then(resolve) {
+                            setImmediate(() => {
+                                console.log("foo");
+                                resolve();
+                            });
+                        },
+                    };
+                } catch (e) {
+                    console.log("FAIL", e);
+                }
+            }
+            async function g() {
+                try {
+                    await f();
+                } catch (e) {}
+            }
+            await g();
+            console.log("bar");
+        })();
+    }
+    expect: {
+        (async function() {
+            await async function() {
+                try {
+                    await async function() {
+                        try {
+                            await {
+                                then(resolve) {
+                                    setImmediate(() => {
+                                        console.log("foo");
+                                        resolve();
+                                    });
+                                },
+                            };
+                        } catch (e) {
+                            console.log("FAIL", e);
+                        }
+                    }();
+                } catch (e) {}
+            }();
+            console.log("bar");
+        })();
+    }
+    expect_stdout: [
+        "foo",
+        "bar",
+    ]
+    node_version: ">=8"
+}