This introduces some tablegen helpers for defining compatibility
warnings. The main aim of this is to both simplify adding new
compatibility warnings as well as to unify the naming of compatibility
warnings.
I’ve refactored ~half of the compatiblity warnings (that follow the
usual scheme) in `DiagnosticSemaKinds.td` for illustration purposes and
also to simplify/unify the wording of some of them (I also corrected a
typo in one of them as a drive-by fix).
I haven’t (yet) migrated *all* warnings even in that one file, and there
are some more specialised ones for which the scheme I’ve established
here doesn’t work (e.g. because they’re warning+error instead of
warning+extwarn; however, warning+extension *is* supported), but the
point of this isn’t to implement *all* compatibility-related warnings
this way, only to make the common case a bit easier to handle.
This currently also only handles C++ compatibility warnings, but it
should be fairly straight-forward to extend the tablegen code so it can
also be used for C compatibility warnings (if this gets merged, I’m
planning to do that in a follow-up pr).
The vast majority of compatibility warnings are emitted by writing
```c++
Diag(Loc, getLangOpts().CPlusPlusYZ ? diag::ext_... : diag::warn_...)
```
in accordance with which I’ve chosen the following naming scheme:
```c++
Diag(Loc, getLangOpts().CPlusPlusYZ ? diag::compat_cxxyz_foo : diag::compat_pre_cxxyz_foo)
```
That is, for a warning about a C++20 feature—i.e. C++≤17
compatibility—we get:
```c++
Diag(Loc, getLangOpts().CPlusPlus20 ? diag::compat_cxx20_foo : diag::compat_pre_cxx20_foo)
```
While there is an argument to be made against writing ‘`compat_cxx20`’
here since is technically a case of ‘C++17 compatibility’ and not ‘C++20
compatibility’, I at least find this easier to reason about, because I
can just write the same number 3 times instead of having to use
`ext_cxx20_foo` but `warn_cxx17_foo`. Instead, I like to read this as a
warning about the ‘compatibility *of* a C++20 feature’ rather than
‘*with* C++17’.
I also experimented with moving all compatibility warnings to a separate
file, but 1. I don’t think it’s worth the effort, and 2. I think it
hurts compile times a bit because at least in my testing I felt that I
had to recompile more code than if we just keep e.g. Sema-specific
compat warnings in the Sema diagnostics file.
Instead, I’ve opted to put them all in the same place within any one
file; currently this is a the very top but I don’t really have strong
opinions about this.
This implements the R2 semantics of P0963.
The R1 semantics, as outlined in the paper, were introduced in Clang 6.
In addition to that, the paper proposes swapping the evaluation order of
condition expressions and the initialization of binding declarations
(i.e. std::tuple-like decompositions).
