Fix delete on optional chain

lib/compress/index.js

@@ -2750,16 +2750,16 @@ def_optimize(AST_UnaryPostfix, function(self, compressor) {
 
 def_optimize(AST_UnaryPrefix, function(self, compressor) {
     var e = self.expression;
-    if (self.operator == "delete"
-        && !(e instanceof AST_SymbolRef
-            || e instanceof AST_PropAccess
-            || is_identifier_atom(e))) {
-        if (e instanceof AST_Sequence) {
-            const exprs = e.expressions.slice();
-            exprs.push(make_node(AST_True, self));
-            return make_sequence(self, exprs).optimize(compressor);
-        }
-        return make_sequence(self, [ e, make_node(AST_True, self) ]).optimize(compressor);
+    if (
+        self.operator == "delete" &&
+        !(
+            e instanceof AST_SymbolRef ||
+            e instanceof AST_PropAccess ||
+            e instanceof AST_Chain ||
+            is_identifier_atom(e)
+        )
+    ) {
+        return make_sequence(self, [e, make_node(AST_True, self)]).optimize(compressor);
     }
     var seq = self.lift_sequences(compressor);
     if (seq !== self) {
@@ -4251,7 +4251,16 @@ def_optimize(AST_Sub, function(self, compressor) {
 });
 
 def_optimize(AST_Chain, function (self, compressor) {
-    if (is_nullish(self.expression, compressor)) return make_node(AST_Undefined, self);
+    if (is_nullish(self.expression, compressor)) {
+        let parent = compressor.parent();
+        // It's valid to delete a nullish optional chain, but if we optimized
+        // this to `delete undefined` then it would appear to be a syntax error
+        // when we try to optimize the delete. Thankfully, `delete 0` is fine.
+        if (parent instanceof AST_UnaryPrefix && parent.operator === "delete") {
+            return make_node_from_constant(0, self);
+        }
+        return make_node(AST_Undefined, self);
+    }
     return self;
 });
 

test/compress/nullish.js

@@ -145,3 +145,33 @@ nullish_coalescing_parens: {
         "PASS",
     ]
 }
+
+delete_nullish: {
+    input: {
+        delete obj?.key;
+        const other = { key: true };
+        delete other?.key;
+    }
+    expect: {
+        delete obj?.key;
+        const other = { key: true };
+        delete other?.key;
+    }
+}
+
+delete_nullish_2: {
+    options = {
+        defaults: true,
+        evaluate: true,
+        passes: 3,
+    }
+    input: {
+        delete null?.key;
+        delete null?.deep.key;
+        delete null.deep?.key;
+        delete null?.deep?.key;
+    }
+    expect: {
+        delete null.deep?.key;
+    }
+}