fix corner case in evaluate (#3921)

fixes #3920

lib/compress.js

@@ -3256,11 +3256,21 @@ merge(Compressor.prototype, {
                     && unaryPrefix[this.operator];
             }
         });
-        function modified(sym) {
-            if (!(sym instanceof AST_SymbolRef)) return;
-            sym.definition().references.forEach(function(node) {
-                delete node._eval;
-            });
+        var scan_modified = new TreeWalker(function(node) {
+            if (node instanceof AST_Assign) modified(node.left);
+            if (node instanceof AST_Unary && unary_arithmetic[node.operator]) modified(node.expression);
+        });
+        function modified(node) {
+            if (node instanceof AST_Dot) {
+                modified(node.expression);
+            } else if (node instanceof AST_Sub) {
+                modified(node.expression);
+                node.property.walk(scan_modified);
+            } else if (node instanceof AST_SymbolRef) {
+                node.definition().references.forEach(function(ref) {
+                    delete ref._eval;
+                });
+            }
         }
         def(AST_Statement, function() {
             throw new Error(string_template("Cannot evaluate a statement [{file}:{line},{col}]", this.start));
@@ -3280,12 +3290,11 @@ merge(Compressor.prototype, {
         });
         def(AST_Sequence, function(compressor, ignore_side_effects, cached, depth) {
             if (!ignore_side_effects) return this;
-            var tail = this.tail_node();
-            this.walk(new TreeWalker(function(node) {
-                if (node === tail) return true;
-                if (node instanceof AST_Assign) modified(node.left);
-                if (node instanceof AST_Unary && unary_arithmetic[node.operator]) modified(node.expression);
-            }));
+            var exprs = this.expressions;
+            for (var i = 0, last = exprs.length - 1; i < last; i++) {
+                exprs[i].walk(scan_modified);
+            }
+            var tail = exprs[last];
             var value = tail._eval(compressor, ignore_side_effects, cached, depth);
             return value === tail ? this : value;
         });

test/compress/evaluate.js

@@ -2463,3 +2463,25 @@ issue_3905: {
     }
     expect_stdout: "0"
 }
+
+issue_3920: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a = function(b) {
+            return (b[b = 0] = 0) >= (b ? 0 : 1);
+        }("foo");
+        console.log(a);
+    }
+    expect: {
+        var a = function(b) {
+            return (b[b = 0] = 0) >= (b ? 0 : 1);
+        }("foo");
+        console.log(a);
+    }
+    expect_stdout: "false"
+}