It's clever that this is short, but using a RegExp is maybe a little obscure? Also typeof x === {aLiteral} is hghly optimized in JS VMs.

What about:

const staticBrand = Symbol();
export const unsafeStatic = (value: string): StaticValue => ({
  ['_$litStatic$']: value,
  r: staticBrand,
});

Then:

if ((typeof (value as Partial<StaticValue>)?.r !== 'symbol')) {

IE11 doesn't have native Symbol, right, so typeof Symbol() === 'symbol' can't work, can it? https://caniuse.com/mdn-javascript_builtins_symbol

We already require the Symbol polyfill.

Symbol.for() might be good like you mentioned before too, then you can do (value as Partial<StaticValue>)?.r === staticBrand

Since these are each brand new instances of RegExp, and not a shared value, I'm assuming you're just trying to guard against JSON injections. In that case, you can take advantage of constructor: undefined to get the same benefits without needing an object (or a instanceof check, which is very slow). JSON cannot inject an undefined value, and a non-undefined constructor is present on all objects, so value.constructor === undefined guarantees the value was not constructed from JSON.

That's a really cool trick @jridgewell! Ended up going with Symbol.for('') for clarity and to avoid hitting VM optimization edge cases.

Oh, erp, didn't push all my changes, should be there now