enhance `dead_code` (#3907)

2 files changed, 30 insertions, 0 deletions

diff --git a/lib/compress.js b/lib/compress.js
index d378e26e..f91d213e 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -7739,6 +7739,10 @@ merge(Compressor.prototype, {
         if (compressor.option("dead_code")) {
             if (self.left instanceof AST_PropAccess) {
                 if (self.operator == "=") {
+                    if (self.left.equivalent_to(self.right)
+                        && (self.left instanceof AST_Dot || !self.left.property.has_side_effects(compressor))) {
+                        return self.right;
+                    }
                     var exp = self.left.expression;
                     if (exp instanceof AST_Lambda
                         || !compressor.has_directive("use strict")
@@ -7751,6 +7755,7 @@ merge(Compressor.prototype, {
                     }
                 }
             } else if (self.left instanceof AST_SymbolRef) {
+                if (self.operator == "=" && self.left.equivalent_to(self.right)) return self.right;
                 if (self.left.is_immutable()) return strip_assignment();
                 var def = self.left.definition();
                 var scope = def.scope.resolve();
diff --git a/test/compress/dead-code.js b/test/compress/dead-code.js
index a1fb9483..795d1f7c 100644
--- a/test/compress/dead-code.js
+++ b/test/compress/dead-code.js
@@ -1168,3 +1168,28 @@ redundant_assignments: {
     }
     expect_stdout: "PASS PASS"
 }
+
+self_assignments: {
+    options = {
+        dead_code: true,
+    }
+    input: {
+        var a = "PASS", b = 0, l = [ "FAIL", "PASS" ], o = { p: "PASS" };
+        a = a;
+        l[0] = l[0];
+        l[b] = l[b];
+        l[b++] = l[b++];
+        o.p = o.p;
+        console.log(a, b, l[0], o.p);
+    }
+    expect: {
+        var a = "PASS", b = 0, l = [ "FAIL", "PASS" ], o = { p: "PASS" };
+        a;
+        l[0];
+        l[b];
+        l[b++] = l[b++];
+        o.p;
+        console.log(a, b, l[0], o.p);
+    }
+    expect_stdout: "PASS 2 PASS PASS"
+}