[PATCH] D75572: [Sema][SVE] Reject sizeof and alignof for sizeless types

[Richard Sandiford via Phabricator via cfe-commits]

rsandifo-arm added a comment.

In D75572#1904415 <https://reviews.llvm.org/D75572#1904415>, @efriedma wrote:

> The planned changes to RequireCompleteType should catch this, right?  If we want a specialized diagnostic for sizeless types in RequireCompleteType, we should probably just pass it to RequireCompleteType.  (Otherwise, you'll end up with the "wrong" diagnostic in templates.)


Yeah, the planned changes to RequireCompleteType would catch this by default.  If RequireCompleteType should always get the first shot, I guess there are three possible approaches:

1. Stick with the approach in D62962 <https://reviews.llvm.org/D62962>, without specialised error messages.  I can break that patch up into smaller chunks if the general approach seems right.
2. Stick with the approach in this patch of having seperate isSizelessType checks and diagnostics, but test isSizelessType after RequireCompleteType instead of before it.  The later changes to RequireCompleteType allow sizeless types to be let through on demand, and the isSizelessType handling would be similar to the isFunctionType handling just below.
3. Like you say, stick with separate diagnostics for incomplete and sizeless types but make RequireCompleteType emit them both.

I guess one way doing 3. would be to use %select{incomplete|sizeless} and pass the result of isSizelessType() as one of the diagnostic parameters to RequireCompleteType.  But in some ways that feels a bit clunky because the caller is then doing the work to prove that the use is invalid (which it always is if isSizelessType()) but isn:t actually doing the work to emit the associated diagnostic.  Another option would be to make RequireCompleteType have a choice between three actions:

- the normal, standard-defined behaviour, with no special error messages for sizeless types.  This would remain the default if no action is explicitly specified.
- a mode that explicitly allows sizeless types
- a mode that explicitly disallows sizeless types, with the first diagnostic parameter being whether the type is sizeless or not.  In contrast to the above, RequireCompleteType would add this parameter automatically.


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