From 4868c73d5bc5eb3ffbbc2fd31c485e3e22795550 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Donny/=EA=B0=95=EB=8F=99=EC=9C=A4?= <kdy1997.dev@gmail.com>
Date: Sat, 9 Apr 2022 03:08:31 +0900
Subject: [PATCH] fix(es/optimization): Don't create invalid sequence
 expressions (#4285)

---
 .../src/simplify/branch/mod.rs                | 25 ++++++++++++++++++-
 .../tests/simplify.rs                         | 18 +++++++++++++
 2 files changed, 42 insertions(+), 1 deletion(-)

diff --git a/crates/swc_ecma_transforms_optimization/src/simplify/branch/mod.rs b/crates/swc_ecma_transforms_optimization/src/simplify/branch/mod.rs
index aaa6c948430..73d0a7187f8 100644
--- a/crates/swc_ecma_transforms_optimization/src/simplify/branch/mod.rs
+++ b/crates/swc_ecma_transforms_optimization/src/simplify/branch/mod.rs
@@ -82,6 +82,12 @@ impl VisitMut for Remover {
         e.visit_mut_children_with(self);
 
         match e {
+            Expr::Seq(s) => {
+                if s.exprs.is_empty() {
+                    *e = Expr::dummy();
+                }
+            }
+
             Expr::Assign(AssignExpr {
                 op: op!("="),
                 left: PatOrExpr::Pat(l),
@@ -154,6 +160,19 @@ impl VisitMut for Remover {
                 *e = *cond.cons.take();
             }
 
+            Expr::Bin(be) => match (be.left.is_invalid(), be.right.is_invalid()) {
+                (true, true) => {
+                    *e = Expr::dummy();
+                }
+                (true, false) => {
+                    *e = *be.right.take();
+                }
+                (false, true) => {
+                    *e = *be.left.take();
+                }
+                _ => {}
+            },
+
             _ => {}
         }
     }
@@ -189,7 +208,7 @@ impl VisitMut for Remover {
         if cfg!(feature = "debug") {
             debug!("Removing dead branches");
         }
-        self.fold_stmt_like(n)
+        self.fold_stmt_like(n);
     }
 
     fn visit_mut_object_pat(&mut self, p: &mut ObjectPat) {
@@ -1483,6 +1502,10 @@ fn ignore_result(e: Expr) -> Option<Expr> {
 
             exprs.extend(last);
 
+            if exprs.is_empty() {
+                return None;
+            }
+
             Some(Expr::Seq(SeqExpr { span, exprs }))
         }
 
diff --git a/crates/swc_ecma_transforms_optimization/tests/simplify.rs b/crates/swc_ecma_transforms_optimization/tests/simplify.rs
index 17e85ca00c2..c3c782397c6 100644
--- a/crates/swc_ecma_transforms_optimization/tests/simplify.rs
+++ b/crates/swc_ecma_transforms_optimization/tests/simplify.rs
@@ -631,6 +631,24 @@ test!(
     "
 );
 
+test!(
+    Syntax::default(),
+    |_| dead_branch_remover(),
+    issue_4272,
+    "
+    function oe() {
+        var e, t;
+        return t !== i && (e, t = t), e = t;
+    }
+    ",
+    "
+    function oe() {
+        var e, t;
+        return e = t;
+    }
+    "
+);
+
 test!(
     Syntax::default(),
     |_| chain!(