Misleading error message for ?? throw.

[lrhn]

The expression

  x ?? throw new ArgumentError("NOooooooooooo....!");

gives the error message:

Error: Expected an expression, but got 'throw'.                                 
  x ?? throw new ArgumentError("NOooooooooooo....!");                           
       ^^^^^                                                                    
Error: Compilation failed.                                                      

This is misleading since throw is an expression. A more correct error message would be:

"throw" is not allowed at this point. Try wrapping the "throw" expression in parentheses.

[zoechi]

"Try wrapping" sounds weird, like try a few things until you stumble upon something that doesn't produce an error.
Why is throw not allowed? What does wrapping in parentheses change?

[lrhn]

Agree on wording. It's a suggestion, so it should't dictate that you do it, but "Maybe you want to wrap ..." is better than "Try wrapping".

The grammar production for ?? is logicalOrExpression '??' logicalOrExpression. A throwExpression is not reachable from a logicalOrExpression. That means that x ?? throw 0 is not valid Dart syntax - the grammar productions do not allow it. Wrapping throw 0 in parentheses changes it from being a throwExpression to being a plain expression which is reachable from logicalOrExpression.

It still sucks. I think the ?? operator should have the same precedence as the ?: operator, but even that would not help here because the throw operator binds very weakly (which was really a mistake and you can blame me for it).

[zoechi]

"Maybe you want..." is definitely better, or "Wrapping the throw expression in parentheses would create an allowed expression."

[zoechi]

@lrhn You say that ?? should have a different precedence and throw binds weakly.
Are there plans to fix this in Dart 2.0? Do you maintain a list of such things that should be fixed?
No need to publish it (would be nice though ;-) ), just want to know if you keep track of these things.

[lrhn]

Not saying anything about ??, it's probably fine as-is - not exactly the same precedence as ?: (which is hard to talk about the precedence of anyway since it differs between the three operands), but it's right above it in the precedence table which is also fine, and possibly even better. That way x ?? y ? z ?? v : w ?? q isn't ambiguous.

I'm saying that throw binds weakly, and it shouldn't have, but it's a limited problem because I don't think most uses of throw depends on the precedence (who writes throw 3 + 4 anyway?).

So, no current plans to change it that I'm aware of.

I don't think x ?? throw ... is particularly recommendable code, and I'd request anyone writing to go back to if (x == null) throw ...;. Don't use an expression to do a statement's job. If you don't want the value from the expression, don't use an expression.

[zoechi]

I see, thanks for your explanation 👍

[askeksa-google]

This is a good suggestion which is still relevant.

[eernstg]

Also note that we could allow e1 ?? throw e2, cf. dart-lang/language#644 (comment). If it's not an error then the choice of error message is less important. ;)