Grammar allows topic in some assignment targets

[js-choi]

@JLHwung raised a concern in babel/babel#13536 (comment). To prevent cluttering up babel/babel#13536, I’m opening an issue here. The topic token is assumed to be % in this issue.

The current grammar does not correctly raise early errors for x |> ((%) => {}) and x |> (async (%) => {}). This is because, inside of a PipeBody, % is hidden from the Contains operation—including in any CoverParenthesizedExpressionAndArrowParameterList that is within the PipeBody.

So, just like how the current grammar (correctly) allows x |> ((%)), so too does it (incorrectly) allow x |> ((%) => {}). In both cases, PipeBody contains a topic inside of two nested CoverParenthesizedExpressionAndArrowParameterLists.

@lightmare pointed to AssignmentTargetType as a good place to prohibit the topic for these cases. This should also let expressions like x |> %[y] = z, x |> y[%] = z, and x |> ((y = %) => {}) remain correctly valid.

(@JLHwung also said that the current grammar also does not correctly raise early errors for x |> (% = 0) and x |> (% += 0). However, I’m not sure if this is true. LeftHandSideExpression allows only NewExpression, CallExpression, and OptionalExpression, and none of these expressions can result in an assignment to a topic reference.)

[lightmare]

so too does it (incorrectly) allow x |> ((%) => {})

I don't think it does. ArrowParameters only allow BindingIdentifier, which % is not.

LeftHandSideExpression allows only NewExpression

NewExpression > MemberExpression > PrimaryExpression > %

[js-choi]

@lightmare: Thanks for the correction; I was mistaken. The grammar currently correctly prohibits x |> ((%) => {}) ((%) matches neither BindingIdentifier nor ( UniqueFormalParameters )) but incorrectly allows x |> (% = 0). Either way, we should add a restriction to AssignmentTargetType as you suggested.