Fwiw this can be reproduced also with an @if block.

@let testType = test.type;
@if(testType == 'a') {
  {{test.name}} // errror 
}

So it looks like this one might me be somehow a typescript issue (a bit like #59215).

The TCB generates something like this

function _tcb1(this: { test: Test }) {
    if (true) {
        const _t1 = (((((this).test)).type));
        if (_t1 === "a") {
            (((((this).test)).name));
        }
    }
} 

The parenthesis seems to mess up the narrowing as you can see in this playground.

Playground demo.

Edit: Upstream issue microsoft/TypeScript#61784

@JeanMeche well guess I need to subscribe to the typescript issue :)
I always wonder why the TCB generate so many parenthesis... Seems redundant

I might need to defer to @JoostK (who is so much more knowledgable on that topic) for this question.

We add comments after each node so we can map the error back to its source. The parentheses ensure that the comment gets associated with the intended node in the AST.

Ah that makes sense... Thanks for the explanation :)

Indeed, that is the reason. There's some more comments, e.g. to suppress diagnostics that would have been reported elsewhere (we sometimes have to repeat certain expression) and some comments specifically for the language service to identify code.

For source mapping you'd think that we could leverage source maps, and that would be a possibility except that TypeScript's ts.Printer API doesn't generate those.