Ban old-style type assertions under erasableSyntaxOnly

[jakebailey]

Code like ()=><any>{} is not erasable, since ()=> {} is a different tree, and there's nowhere to add parens to the body without shifting its contents backwards one character. If there were extra spaces after the {}, maybe it'd work, but that's atypical.

This PR bans this specific case, though, I personally feel like banning the old-style assertions would be a good idea too.

Updated the PR to just ban them. There are too may edge cases IMO.

[ajafff]

you probably want to ban it in more places:

// return and yield ASI
function *foo() {
    yield <any>
        1;
    return <any>
        1;
}

// at the start of an ExpressionStatement if followed by an object literal; though I'm not sure why one would use it there
<unknown>{foo() {}}.foo();

// at the start of an ExpressionStatement if followed by function keyword
<unknown>function() {}();
<unknown>function() {};

// at the start of an ExpressionStatement if followed by an anonymous class expression
// note that this exact syntax currently emits invalid JS (no parenthesis added like for function above)
<unknown>class {}

there's probably more parsing ambiguity I cannot remember right now.

[jakebailey]

Yeah, so that furthers my feeling that this syntax should be wholly banned in this mode. I tried to carve something out, but it sure doesn't seem like that's tractable if we want to ban something here.

[acutmore]

When I was first creating ts-blank-space I tried to write the logic that only errors on the 'actual' bad cases but ended up coming to the conclusion that this may be more confusing to try and explain. This is why I ended up always erroring on the old style of type assertions and recommended code replaces them with as style assertions. The extra win of migrating to as is that code can more easily be moved to a .tsx file without getting sometimes cryptic errors.

[jakebailey]

Updated the PR to just straight up ban the syntax.

[jakebailey]

@typescript-bot cherry-pick this to release-5.8

[typescript-bot]

Starting jobs; this comment will be updated as builds start and complete.

Command	Status	Results
cherry-pick this to release-5.8	✅ Started	✅ Results

[typescript-bot]

Hey, @jakebailey! I've created #61320 for you.