If top-level await is only valid in modules, should seeing one set sawUnambiguousESM? In other words, how should this behave with a sourceType of unambiguous? Currently, parsing await 0 with unambiguous yields an AST with sourceType of script, even though if you'd specified script it would not have parsed:

> require('./').parse('await 0', { plugins: ['topLevelAwait'], sourceType: 'unambiguous' }).program.sourceType
'script'
> require('./').parse('await 0', { plugins: ['topLevelAwait'], sourceType: 'script' }).program.sourceType
Thrown:
{ [SyntaxError: Unexpected token, expected ";" (1:6)
] pos: 6, loc: Position { line: 1, column: 6 } }

This is tricky because, for exmaple, this is both a valid script and a valid module:

await
0

I can set it to a module when await the next invalid token is on the same line.

This is a script that contains an await expression? I thought that wasn’t allowed.

We allow it using the allowAwaitOutsideFunction option, but it's not in the spec.
It's used, for example, by @babel/template users who might then wrap it in an async function.

I think the following examples are also ambiguous

await + 0
await - 0
await ( 0 )
await [ 0 ]
await / 0 /u

I am not sure if they are complete but we can always lookahead charcode to fix them.

Awesome! I missed await ` 0 ` .

I come up with a new case: If one turns on v8intrinsic, the following could be ambiguous, too.

await %a()