Difference in output for DECLARES_INSTANCE_FIELD in extension and extension type

[FMorschel]

I was working on https://dart-review.googlesource.com/c/sdk/+/402622 and I got to this code:

extension E on int {
  final int foo = 1, bar = 2;
}

Which has a similar case as:

extension type A(int i) {
  final int foo = 1, bar = 2;
}

But we have different outputs here:

To start, the error on extension type is a CompileTimeErrorCode and the error on extension is a ParserErrorCode. I'm unsure of what is the differentiation between them but I don't see any semantic differentiation here that would make them different.

But the problem I opened this to discuss is actually the following:

Why do we get more errors on one than on the other? That doesn't seem coherent to me.

CC @bwilkerson @scheglov @srawlins

[bwilkerson]

A ParserErrorCode is used for errors produced by the parser; a CompileTimeErrorCode is used for errors produced by later stages of the compiler. From a semantic perspective they're both errors and hence there's no interesting difference. Both will prevent code from being compiled.

Yes, the differences in reporting these diagnostics is a bit odd. I suspect that the only reason they're different is because they're being reported in different pieces of code written by different people. There's certainly no inherent reason for the differences.

[FMorschel]

Should we keep this open to make the error on extension to trigger for all variables?

[bwilkerson]

Yes, I think triggering on every variable is the better approach because it's also more consistent with what we'd do if each field were declared in a separate declaration.