@Kingwl you might be interested in this one.

It’s not just String.raw, this applies to all tagged template literals, where the top‑level TemplateStringArray contains undefined for an unknown escape, so this should also update TemplateStringsArray:

const taggedTemplate = (template: TemplateStringsArray, ...substitutions: unknown[]) => {
	console.log(template, substitutions);
}

// Logs:
// [
// 	undefined, "bar", undefined,
// 	raw: [ "foo\\uvar", "bar", "baz\\u" ]
// ]
// [ 123, 456 ]
taggedTemplate`foo\uvar${123}bar${456}baz\u`;

Workbench Repro

Also, this doesn’t occur when the \u follows a substitution.

It’s not just String.raw, this applies to all tagged template literals

Sorry I'm not 100% sure what your means.
But seems the runtime behavior is as expected.

The current problem is falsely alarm.

Here's some context:

• #23801
• https://tc39.github.io/proposal-template-literal-revision/

And I have found an PR who try to fix a part of the alarm: #41030

The definition of TemplateStringArray should include undefined in the top‑level index signature, which should be fixed alongside this to correctly describe the runtime behaviour as of tc39/ecma262@138c9ef.

Ahhhh Okay. Got the point. Thanks.

👋 Hi, I'm the Repro bot. I can help narrow down and track compiler bugs across releases! This comment reflects the current state of the repro in this issue running against the nightly TypeScript.

❌ Failed: -

• Hexadecimal digit expected.

I think that dealing with octal escapes in all strings #396 should be done before dealing with tagged templates as an exception...