TypeScript: x < y >= z is incorrectly interpreted as a type argument list

[evanw]

Here's a full example (link):

// Input
fn(x < y, x >= y)

// Sucrase
fn(x = y)

// TypeScript
fn(x < y, x >= y);

Specifically TypeScript's parseTypeArgumentsInExpression function backtracks unless the trailing > comes from a > real token. This is subtle because TypeScript's type argument parser normally strips off the leading > from a token, which is why >> works in a<b<c>>(d). But it turns out that should only be done when you're in a type context. When you're in an expression context, the trailing > must not come from a token that has anything else after it. So for example a<b<c>>=(d) should not be considered a type argument list because parseTypeArgumentsInExpression encounters a >= token.

I just fixed this bug in esbuild: evanw/esbuild#3111. I'm reporting it here because Sucrase appears to also have this issue. The bug does not introduce a syntax error, which means the problematic output might be missed before releasing it, so I considered this bug to be higher severity.

[alangpierce]

Thank you letting me know! Agreed that it's higher severity.