fix corner case in merge_vars (#5092)

fixes #5091

lib/compress.js

@@ -5672,7 +5672,10 @@ merge(Compressor.prototype, {
             if (node instanceof AST_Call) {
                 var exp = node.expression;
                 var tail = exp.tail_node();
-                if (!(tail instanceof AST_LambdaExpression)) return walk_node_with_expr(node);
+                if (!(tail instanceof AST_LambdaExpression)) {
+                    descend();
+                    return mark_expression(exp);
+                }
                 if (exp !== tail) exp.expressions.slice(0, -1).forEach(function(node) {
                     node.walk(tw);
                 });
@@ -5788,7 +5791,18 @@ merge(Compressor.prototype, {
                 pop();
                 return true;
             }
-            if (node instanceof AST_Sub) return walk_node_with_expr(node);
+            if (node instanceof AST_Sub) {
+                var exp = node.expression;
+                if (node.optional) {
+                    exp.walk(tw);
+                    push();
+                    node.property.walk(tw);
+                    pop();
+                } else {
+                    descend();
+                }
+                return mark_expression(exp);
+            }
             if (node instanceof AST_Switch) {
                 node.expression.walk(tw);
                 var save = segment;
@@ -5888,10 +5902,9 @@ merge(Compressor.prototype, {
                 return true;
             }
 
-            function walk_node_with_expr(node) {
+            function mark_expression(exp) {
-                descend();
                 if (compressor.option("ie")) {
-                    var sym = root_expr(node.expression);
+                    var sym = root_expr(exp);
                     if (sym instanceof AST_SymbolRef) sym.walk(tw);
                 }
                 return true;

test/compress/optional-chains.js

@@ -327,3 +327,29 @@ issue_5039: {
     expect_stdout: "PASS"
     node_version: ">=14"
 }
+
+issue_5091: {
+    options = {
+        merge_vars: true,
+    }
+    input: {
+        function f(a) {
+            var b = a.p;
+            var c;
+            b?.[c = "FAIL 2"];
+            return b || c;
+        }
+        console.log(f("FAIL 1") || "PASS");
+    }
+    expect: {
+        function f(b) {
+            var b = b.p;
+            var c;
+            b?.[c = "FAIL 2"];
+            return b || c;
+        }
+        console.log(f("FAIL 1") || "PASS");
+    }
+    expect_stdout: "PASS"
+    node_version: ">=14"
+}