Make TS InstantiationExpr parsing more permissive

[g-plane]

This PR allows:

const x4 = f<true>
if (true) {}

Previously it's an error.

References:

• ASI not working after arguments-less new expression with type arguments microsoft/TypeScript#48654
• More permissive parsing of instantiation expressions microsoft/TypeScript#48659

[evanw]

Thanks for giving me a heads up about this change in TypeScript!

[evanw]

Where did these come from? This doesn't appear to follow what Microsoft changed in their parser. For example, consider the following code:

f<T>
import "x"

f<T>
import("x")

After their PR, TypeScript parses it like this:

f;
import "x";

f < T > import("x");

But with your PR, esbuild parses it like this:

f;
import "x";
f;
import("x");

I tested this case with import because some of the code Microsoft is now using looks like this:

function isStartOfLeftHandSideExpression(): boolean {
  switch (token()) {
    case SyntaxKind.ThisKeyword:
    case SyntaxKind.SuperKeyword:
    case SyntaxKind.NullKeyword:
    case SyntaxKind.TrueKeyword:
    case SyntaxKind.FalseKeyword:
    case SyntaxKind.NumericLiteral:
    case SyntaxKind.BigIntLiteral:
    case SyntaxKind.StringLiteral:
    case SyntaxKind.NoSubstitutionTemplateLiteral:
    case SyntaxKind.TemplateHead:
    case SyntaxKind.OpenParenToken:
    case SyntaxKind.OpenBracketToken:
    case SyntaxKind.OpenBraceToken:
    case SyntaxKind.FunctionKeyword:
    case SyntaxKind.ClassKeyword:
    case SyntaxKind.NewKeyword:
    case SyntaxKind.SlashToken:
    case SyntaxKind.SlashEqualsToken:
    case SyntaxKind.Identifier:
      return true;
    case SyntaxKind.ImportKeyword:
      return lookAhead(nextTokenIsOpenParenOrLessThanOrDot);
    default:
      return isIdentifier();
  }
}

[g-plane]

I forgot to handle import token, and I will add it.

They changed to use an existing function isStartOfExpression.

[g-plane]

How can I do something similar to lookAhead in esbuild?

[evanw]

It's kind of hacky. I stash a copy of p.lexer on the stack and then overwrite it again after backtracking has finished. I can take this PR from here and add the backtracking.

[g-plane]

To be honest, I hope we can have a more common method to do backtracking.