Actually enable -preview=fieldwise by default

[pbackus]

This change was accidentally left out of PR #17495.

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[pbackus]

@ryuukk Workaround is to define a BetterC-compatible opEquals for the affected type (in this case, that would be typeof(item)).

[pbackus]

Unfortunately this looks like it opens a huge can of worms, so maybe better to delay for now.

[pbackus]

The == operator should not behave differently in BetterC than it does in regular D.

[pbackus]

Closing in favor of #20672

[dkorpel]

@ryuukk Do you have a self-contained test case for that error? Adding that to the test suite would prevent accidentally breaking your project in the future.

[pbackus]

@dkorpel The error happens because with -preview=fieldwise, an == test is performed that was originally (erroneously) skipped. That == test (items[i].attributes == item.attributes) was always broken in BetterC, and if the project had ever attempted to perform it directly, it would have triggered the exact same error, with or without this PR.

[dkorpel]

What are the types of items[i].attributes and item.attributes?

[pbackus]

Relevant issue is #18197