fix corner case in reduce_vars (#3975)

fixes #3974

lib/compress.js

@@ -4032,7 +4032,9 @@ merge(Compressor.prototype, {
             return all(this.elements);
         });
         def(AST_Binary, function() {
-            return this.left.is_constant_expression() && this.right.is_constant_expression();
+            return this.left.is_constant_expression()
+                && this.right.is_constant_expression()
+                && (this.operator != "in" || is_object(this.right.tail_node()));
         });
         def(AST_Constant, return_true);
         def(AST_Lambda, function(scope) {

test/compress/reduce_vars.js

@@ -7358,3 +7358,28 @@ issue_3958: {
         "0",
     ]
 }
+
+issue_3974: {
+    options = {
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        try {
+            var a = 0 in 0;
+            0 && a;
+        } catch (e) {
+            console.log("PASS");
+        }
+    }
+    expect: {
+        try {
+            var a = 0 in 0;
+            0 && a;
+        } catch (e) {
+            console.log("PASS");
+        }
+    }
+    expect_stdout: "PASS"
+}