https://github.com/AaronBallman commented:
Thank you for working on this!
I think there may be a way we can implement this by updating the handling of
`%diff` in `FormatDiagnostic()` in `Diagnostic.cpp`. That syntax only accepts
two `QualType` objects and is used when we're going to report a diagnostic
about a type mismatch between the two. So any such diagnostic could plausibly
be handed two VLA types and run into the same sort of confusing output for the
user.
I can think of two ways to do this (other ways may exist).
1) We could add new syntax for `%diff` to let the diagnostic author pick a
custom string when the two types would print identical output. e.g.,
`%diff{assigning $ to a variable of type $|assigning with different
types|assigning with different VLA types $}0,1`
2) We could be clever and generate text when formatting the diagnostic. e.g.,
notice that the output of the types would be identical and append `; VLA types
differ despite using the same array size expression` or something along those
lines.
I worry that #2 will wind up with awkward diagnostic wording that seems magical
to diagnostic authors because it won't be immediately clear where the wording
comes from. I worry that #1 doesn't scale well and forces everyone to think
about VLAs when it may not be clear to them how to work such a diagnostic. So
my idea may be a terrible one, and if so, feel free to tell me. ;-)
https://github.com/llvm/llvm-project/pull/101261
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