[PATCH] D125178: Warn if using `elifdef` & `elifndef` in not C2x mode

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman added inline comments.


================
Comment at: clang/include/clang/Basic/DiagnosticLexKinds.td:696-698
+def ext_c2x_pp_directive : Extension<
+  "%select{#elifdef|#elifndef}0 is a C2x extension">,
+  InGroup<CPre2xCompatPedantic>;
----------------
ken-matsui wrote:
> aaron.ballman wrote:
> > ken-matsui wrote:
> > > aaron.ballman wrote:
> > > > I think we need two diagnostics, one for C2x and one for C++2b (https://wg21.link/p2334 was adopted for C++23). Each of these diagnostics should come in a pair:
> > > > ```
> > > > def warn_cxx20_compat_pp_directive : Warning<
> > > >   "use of a '#%select{elifdef|elifndef}0' directive is incompatible with C++ standards before C++2b",
> > > >   InGroup<CXXPre2bCompat>, DefaultIgnore;
> > > > def ext_cxx20_pp_directive : ExtWarn<
> > > >   "use of a '#%select{elifdef|elifndef}0' directive is a C++2b extension",
> > > >   InGroup<CXX2b>;
> > > > ```
> > > > and similar for C, except with wording about C standards and in the C warning groups.
> > > I thought I had to use `Extension` here, but what is the difference between `Warning`, `ExtWarn`, and `Extension`?
> > Great question!
> > 
> > `Extension` diagnostics are warnings that are enabled via `-pedantic` but otherwise off by default.
> > `ExtWarn` diagnostics are warnings that are enabled via `-pedantic` but are also on by default.
> > `Warning` diagnostics are warnings. You can use things like `DefaultIgnore` or `DefaultError` to control the default behavior of the warning.
> > `Error` diagnostics are errors.
> > 
> > You'll typically use `Extension` or `ExtWarn` for things that are extensions of some sort, but which one you use depends on how important the warning is. We've traditionally handled "use of whatever is a C++NN extension" warnings as `ExtWarn` whereas the "use of whatever is incompatible with C++ standards before C++NN" warnings are usually `Extension`.
> In this case, `compat_pp_directive` for C++2b & C2x uses `Warning`, but why did you choose it rather than `Extension`?
Oh shoot, I misspoke and said `ExtWarn` when I meant `Warning` that's default ignored. Sorry about that, and thank you for asking for clarification!

I picked that because it's the pattern used by many other diagnostic pairs:
```
def ext_inline_variable : ExtWarn<
  "inline variables are a C++17 extension">, InGroup<CXX17>;
def warn_cxx14_compat_inline_variable : Warning<
  "inline variables are incompatible with C++ standards before C++17">,
  DefaultIgnore, InGroup<CXXPre17Compat>;

def ext_for_range_begin_end_types_differ : ExtWarn<
  "'begin' and 'end' returning different types (%0 and %1) is a C++17 extension">,
  InGroup<CXX17>;
def warn_for_range_begin_end_types_differ : Warning<
  "'begin' and 'end' returning different types (%0 and %1) is incompatible "
  "with C++ standards before C++17">, InGroup<CXXPre17Compat>, DefaultIgnore;

def ext_constexpr_body_invalid_stmt : ExtWarn<
  "use of this statement in a constexpr %select{function|constructor}0 "
  "is a C++14 extension">, InGroup<CXX14>;
def warn_cxx11_compat_constexpr_body_invalid_stmt : Warning<
  "use of this statement in a constexpr %select{function|constructor}0 "
  "is incompatible with C++ standards before C++14">,
  InGroup<CXXPre14Compat>, DefaultIgnore;

def ext_constexpr_function_try_block_cxx20 : ExtWarn<
  "function try block in constexpr %select{function|constructor}0 is "
  "a C++20 extension">, InGroup<CXX20>;
def warn_cxx17_compat_constexpr_function_try_block : Warning<
  "function try block in constexpr %select{function|constructor}0 is "
  "incompatible with C++ standards before C++20">,
  InGroup<CXXPre20Compat>, DefaultIgnore;
```


Repository:
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  https://reviews.llvm.org/D125178/new/

https://reviews.llvm.org/D125178